1 Jul 2026

Cancer treatment has come a long way over the years. While surgery, chemotherapy, radiation, and hormone therapy remain important parts of cancer care, immunotherapy has opened another door for many patients. Instead of directly attacking cancer cells in the same way as traditional chemotherapy, immunotherapy works by helping the body’s own immune system recognize and fight cancer.

One of the most well-known immunotherapy medications is Keytruda (pembrolizumab). For some people, it has become a powerful treatment option, offering hope when cancer is advanced, has returned, or has certain characteristics that make it more likely to respond to immunotherapy. For others, the treatment may provide limited benefit, cause difficult side effects, or introduce new challenges into everyday life.

Like many cancer treatments, Keytruda has both promising benefits and meaningful risks. Although some serious side effects are uncommon, they are real, and they can affect a person’s physical health, emotional well-being, relationships, independence, and quality of life.

Understanding both sides of immunotherapy can help patients and their loved ones have more informed conversations with their healthcare team and prepare for the realities of treatment.

What Is Immunotherapy?

Immunotherapy is a type of cancer treatment that uses the body’s immune system to help fight cancer. Under normal circumstances, the immune system identifies and attacks harmful invaders, such as certain bacteria and viruses. It can also recognize abnormal cells, including some cancer cells.

However, cancer is complicated. Some cancer cells develop ways to hide from the immune system or send signals that prevent immune cells from attacking them. This allows cancer to grow and spread even when the body has natural defenses.

Immunotherapy aims to interfere with those protective mechanisms and help the immune system respond more effectively.

There are several forms of immunotherapy, including immune checkpoint inhibitors, CAR T-cell therapy, certain monoclonal antibodies, and cancer vaccines. These treatments work in different ways, and they are not interchangeable.

Keytruda belongs to a group of medications known as immune checkpoint inhibitors.

How Does Keytruda Work?

Keytruda is a laboratory-made antibody that targets a protein called PD-1, found on certain immune cells called T cells.

PD-1 acts like a brake on the immune system. This braking mechanism is important because it helps prevent the immune system from attacking healthy tissues unnecessarily. Some cancer cells take advantage of this system by interacting with PD-1 through proteins such as PD-L1, helping them evade an immune response.

Keytruda blocks PD-1 from interacting with these proteins. By releasing this immune checkpoint, it can help T cells recognize and attack cancer cells more effectively.

The important distinction is that Keytruda does not work by directly killing cancer cells in the same way many chemotherapy drugs do. It helps the immune system mount a stronger response against cancer.

This approach can be very effective for certain patients. Some experience significant tumor shrinkage, long-lasting disease control, or remission. However, not everyone responds, and researchers are still working to understand why immunotherapy works remarkably well for some people but offers little benefit to others.

Which Cancers Can Keytruda Treat?

Keytruda is used to treat several types of cancer, depending on the disease, its stage, specific tumor characteristics, and whether the medication is used alone or in combination with other treatments.

Examples include:

  • Melanoma, a serious form of skin cancer.
  • Non-small cell lung cancer.
  • Certain types of breast cancer, including some triple-negative breast cancers.
  • Head and neck cancers.
  • Certain bladder and other urinary tract cancers.
  • Kidney cancer in specific treatment settings.
  • Classical Hodgkin lymphoma.
  • Certain cancers of the stomach, esophagus, and other parts of the digestive system.
  • Cervical cancer.
  • Endometrial cancer.
  • Certain colorectal cancers, particularly those with specific molecular features.
  • Some other cancers that have biomarkers such as mismatch repair deficiency (dMMR), high microsatellite instability (MSI-H), or high tumor mutational burden, where applicable.

This list is not exhaustive. Approval depends on the exact diagnosis and treatment setting, and some uses involve specific testing requirements or combination therapies.

For example, in breast cancer, Keytruda may be used with chemotherapy for certain patients with high-risk early-stage triple-negative breast cancer and for some patients with locally recurrent or metastatic triple-negative disease. It is not a standard immunotherapy option for every type of breast cancer.

Before treatment, the oncology team may recommend testing tumor tissue for biomarkers that help determine whether Keytruda is appropriate. These results can help guide decisions, but they do not guarantee that a patient will respond.

The Positive Side of Keytruda: Why It Can Be Life-Changing

For patients who are good candidates, Keytruda can offer benefits that make the risks worth considering. The treatment has changed how certain cancers are managed and has given some people options that were not previously available to them.

1. It can produce lasting responses

One of the most encouraging aspects of immunotherapy is that some patients experience long-lasting cancer control, even after treatment ends. In certain cancers, the immune system may continue recognizing and responding to cancer cells after the medication has been discontinued.

This does not happen for everyone, and a lasting response is not guaranteed. Still, for people who respond well, the possibility of prolonged remission or sustained control of advanced disease is a significant benefit.

2. It may improve outcomes when combined with other treatments

Keytruda is sometimes used alongside chemotherapy, surgery, or other cancer treatments. Depending on the cancer, this combination may improve the chance of controlling the disease, reducing the risk of recurrence, or shrinking tumors before surgery.

For some patients with high-risk early-stage triple-negative breast cancer, for example, a treatment plan may include Keytruda with chemotherapy before surgery, followed by additional Keytruda afterward.

These combinations can also mean that patients experience side effects from more than one treatment, making symptom management especially important.

3. It may be less damaging to some healthy tissues than traditional chemotherapy

Because Keytruda works by altering the immune response rather than directly targeting rapidly dividing cells, its side-effect profile differs from that of conventional chemotherapy.

Some patients do not experience the same degree of hair loss, nausea, or suppression of blood cell production associated with certain chemotherapy regimens. However, these differences should not be mistaken for an absence of side effects. Keytruda can cause significant illness in its own right, and patients receiving combination therapy may still experience typical chemotherapy effects.

4. It offers another treatment option for selected cancers

For people whose cancer has returned, spread, or has particular biological features, immunotherapy may offer an additional approach to treatment.

The benefits depend on the type of cancer, its molecular characteristics, the patient’s overall health, and previous treatments. A medication that is highly effective for one person may not work for another person with the same cancer diagnosis.

The National Cancer Institute provides additional information about how immune checkpoint inhibitors work and which cancers may be treated with them.

The Negative Side: Keytruda Side Effects That Can Affect Daily Life

It is easy to focus on the potential benefits of promising cancer medication, particularly when a person is fighting a serious disease. But patients deserve to understand the other side of the experience, too.

A side effect does not have to be life-threatening to significantly affect someone’s life. Fatigue that prevents someone from working, diarrhea that makes leaving home difficult, or joint pain that interferes with sleep can all have meaningful consequences.

Keytruda side effects vary widely. Some patients experience few problems, while others develop several symptoms or serious complications. Side effects can also appear after treatment has been underway for some time, and certain immune-related problems can emerge after treatment has stopped.

1. Fatigue and exhaustion

Fatigue is one of the more commonly reported side effects of Keytruda. This is not always the kind of tiredness that disappears after a good night’s sleep.

Patients may experience persistent exhaustion, reduced stamina, difficulty concentrating, or a feeling that even small tasks require too much effort.

Everyday activities that once seemed effortless may become challenging, including:

  • Showering, dressing, and preparing meals.
  • Working a full day or keeping up with household responsibilities.
  • Driving to appointments or running errands.
  • Exercising, socializing, or participating in hobbies.
  • Spending time with children, partners, and friends.

Fatigue can also affect emotional health. People may become frustrated when they cannot do the things they used to do or feel guilty when family members take on additional responsibilities.

It is important not to assume that severe fatigue is simply an unavoidable part of cancer treatment. Anemia, poor nutrition, sleep problems, cancer progression, and hormone-producing gland disorders can also cause fatigue. Some of these problems require specific treatment.

2. Skin rashes, itching, and other skin changes

Keytruda can cause skin-related side effects, including itching, dryness, and rashes. In some cases, the skin becomes inflamed or develops more serious reactions.

Even when these symptoms are not medically dangerous, they can be uncomfortable and disruptive.

Persistent itching may interfere with sleep. A rash on the face, neck, chest, or arms may affect self-confidence, body image, and willingness to socialize. Skin discomfort can also make clothing uncomfortable and interfere with daily routines.

More severe skin reactions are possible and require prompt medical evaluation, particularly if a rash spreads quickly, becomes painful, blisters, or involves the mouth or eyes.

3. Diarrhea and inflammation of the digestive system

Diarrhea, nausea, abdominal discomfort, and changes in appetite can occur during Keytruda treatment.

One particularly important complication is immune-mediated colitis, an inflammation of the colon caused by an immune response that affects healthy tissue.

This is different from ordinary digestive upset because it can become serious and may require corticosteroids or other immune-suppressing medication.

Its impact on daily life can be substantial. Patients may:

  • Need to remain close to a bathroom.
  • Avoid restaurants, travel, social events, or long car rides.
  • Experience abdominal cramps, urgency, or disrupted sleep.
  • Become dehydrated or lose weight.
  • Feel embarrassed or anxious about leaving home.

Blood in the stool, severe abdominal pain, or persistent or worsening diarrhea should never be dismissed as a routine treatment inconvenience.

4. Muscle and joint pain

Muscle aches, joint pain, and stiffness can occur with Keytruda. Some patients experience mild discomfort, while others find that pain affects mobility and independence.

Getting out of bed, climbing stairs, standing for long periods, or opening containers may become harder. Pain can limit exercise and interfere with sleep, which can then worsen fatigue and mood.

In some cases, new pain may reflect immune-related inflammation of joints or muscles rather than ordinary aches. Rarely, immune-related muscle inflammation can affect the muscles involved in swallowing or breathing and become a medical emergency.

Patients should report new, persistent, worsening, or unexplained muscle and joint symptoms to their oncology team.

5. Nausea, appetite changes, and weight loss

Some patients develop nausea, reduced appetite, constipation, or abdominal discomfort. These problems may be especially difficult for people who are already struggling to maintain weight or recover from surgery or other cancer treatments.

When eating becomes uncomfortable or food loses its appeal, maintaining adequate nutrition and hydration can be challenging.

A patient may find themselves skipping meals, losing strength, or having less energy to participate in everyday activities. If Keytruda is combined with chemotherapy, the combined treatment plan may produce additional digestive side effects.

Early support from the oncology team or an oncology dietitian can help patients address these problems before they become more severe.

The Less Common but Serious Risks of Keytruda

This is an especially important part of understanding immunotherapy.

Keytruda works by releasing some of the immune system’s natural restraints. That helps the immune system attack cancer, but it can also cause immune cells to attack healthy organs.

These complications are called immune-related adverse events. They may be uncommon compared with everyday side effects, but they can be serious, cause lasting damage, require hospitalization, or occasionally be fatal. They can affect more than one organ at the same time.

Pneumonitis: Inflammation of the lungs

Immune-related pneumonitis occurs when the lungs become inflamed. Symptoms may include a new or worsening cough, shortness of breath, or chest discomfort.

In everyday life, even a relatively mild reduction in breathing capacity can make walking, climbing stairs, showering, or completing household tasks difficult. More serious inflammation can interfere with oxygen exchange and become life-threatening.

Because lung symptoms can also result from infection, cancer, blood clots, or previous radiation therapy, medical evaluation is important rather than assuming the cause.

Colitis: Inflammation of the colon

As discussed earlier, immune-related colitis can cause persistent diarrhea, abdominal pain, and sometimes blood in the stool.

Severe cases can result in dehydration, bleeding, or dangerous complications requiring hospital care. Symptoms that initially seem manageable can worsen, so patients should contact their oncology team promptly when diarrhea develops or changes.

Hepatitis: Inflammation of the liver

Keytruda can cause the immune system to inflame the liver. This may initially show up in blood tests before a patient notices symptoms.

When symptoms do occur, they can include unusual fatigue, nausea, abdominal pain, dark urine, or yellowing of the skin or eyes.

Routine blood testing is one reason oncology teams monitor liver function during treatment. Patients should report concerning symptoms even if their previous blood tests were normal.

Endocrine disorders: Changes in hormone production

The endocrine system includes glands that produce hormones controlling metabolism, energy, temperature regulation, stress responses, and many other functions.

Keytruda can affect the thyroid, pituitary gland, adrenal glands, and, in some cases, the pancreas.

Possible complications include:

  • Hypothyroidism: An underactive thyroid that can contribute to fatigue, feeling cold, constipation, dry skin, and changes in weight or mood.
  • Hyperthyroidism: An overactive thyroid that may cause a racing heartbeat, shakiness, anxiety, sweating, or difficulty sleeping.
  • Adrenal insufficiency: Reduced production of essential stress hormones, potentially causing severe weakness, dizziness, low blood pressure, nausea, or abdominal symptoms.
  • Pituitary inflammation: This can disrupt several hormone systems and cause headaches, fatigue, and other symptoms.
  • Type 1 diabetes: In rare cases, immune-related damage to insulin-producing cells can cause dangerously high blood sugar and diabetic ketoacidosis.

Some hormone problems may be permanent, even when the inflammation that caused them improves. Patients may need long-term hormone replacement, diabetes management, or ongoing monitoring.

This means a person may complete cancer treatment but continue managing a new medical condition for years afterward.

Nephritis: Inflammation of the kidneys

Kidney inflammation can impair the body’s ability to filter waste and maintain fluid and electrolyte balance.

Symptoms are not always obvious at first, which makes blood testing and medical monitoring important. Changes in urination, swelling, or other new symptoms should be reported to a healthcare professional.

Heart and nervous system complications

Rarely, Keytruda can cause inflammation involving the heart, nerves, or other parts of the nervous system.

Potential warning signs include chest pain, an irregular or racing heartbeat, fainting, new muscle weakness, numbness, difficulty swallowing, confusion, or breathing difficulties.

These symptoms require urgent assessment because some immune-related heart and neurological complications can progress rapidly.

Severe infusion reactions

Although many patients tolerate the infusion process, some develop reactions during or shortly after treatment. Symptoms may include chills, fever, flushing, rash, dizziness, wheezing, or difficulty breathing.

Patients should alert the infusion nurse immediately if they experience unusual symptoms during treatment.

The exact risks depend on the patient’s treatment regimen and medical history. The official prescribing information lists additional warnings and explains when treatment may need to be withheld or permanently discontinued.

How Keytruda Side Effects Can Affect Mental Health

The physical effects of cancer treatment are only part of the story. Living with cancer often brings emotional uncertainty, and treatment-related side effects can add another layer of stress.

A patient may feel grateful to have access to immunotherapy while also feeling exhausted, frightened, frustrated, or overwhelmed by what treatment is doing to their body. These emotions can coexist. Someone does not have to feel positive about every part of treatment to appreciate its potential benefits.

Some of the emotional challenges include:

  • Anxiety about symptoms: Every new ache, cough, or change in energy can raise concerns about whether something serious is developing.
  • Fear of the unknown: Patients may worry about whether treatment is working, whether the cancer will return, or whether a side effect could become permanent.
  • Loss of independence: Needing help with daily tasks can be difficult for people who are accustomed to caring for themselves and others.
  • Depression and isolation: Fatigue, pain, and reduced social activity can make a person feel disconnected from friends and family.
  • Frustration with the body: People may feel betrayed by a body that is already dealing with cancer and now seems to be reacting to treatment.
  • Treatment-related uncertainty: Even when scans show improvement, patients may remain anxious about future appointments, laboratory results, and possible complications.

Some people also experience cognitive difficulties, such as trouble concentrating or remembering information. These symptoms can have multiple causes, including cancer itself, poor sleep, stress, medication effects, and hormone changes. New or significant cognitive symptoms should be discussed with the care team.

Emotional support is not a luxury during cancer treatment. Counseling, support groups, oncology social workers, and conversations with trusted loved ones can help patients navigate the emotional burden alongside the physical one.

How Keytruda Can Affect Relationships and Intimacy

Cancer treatment does not happen in isolation. It affects partners, families, friendships, work responsibilities, and the ways people connect with one another.

When side effects interfere with energy, comfort, or emotional well-being, intimacy can change, too.

A person who is constantly tired or experiencing pain may have little interest in sexual activity. Someone dealing with digestive symptoms or unpredictable fatigue may worry about being physically close to a partner. Changes in body image, anxiety, or fear about the future can also affect sexual desire and confidence.

For some people, endocrine complications can influence libido, mood, and general well-being. However, sexual difficulties should not automatically be attributed to Keytruda, because cancer, surgery, chemotherapy, radiation, other medications, and relationship stress may also contribute.

Partners can struggle with these changes as well. One person may want to provide comfort and closeness, while the other needs rest, space, or reassurance. Without communication, both people may misinterpret what is happening.

A few practical approaches can help:

  • Talk openly about physical limitations and emotional needs without assigning blame.
  • Redefine intimacy to include affection, cuddling, massage, emotional closeness, and other forms of connection that feel comfortable.
  • Plan intimate time around energy levels rather than assuming desire and stamina will always be predictable.
  • Discuss persistent sexual concerns with an oncology team, gynecologist, urologist, or qualified sexual-health professional.
  • Seek couples counseling or individual counseling when cancer-related stress begins to affect the relationship.

Intimacy does not have to disappear during cancer treatment, but it may need to look different. Patients deserve support for these concerns just as they deserve help managing pain, nausea, and fatigue.

What Patients Can Do to Manage Side Effects

There is no guaranteed way to prevent every side effect from Keytruda. However, early communication, regular monitoring, and appropriate treatment can make a meaningful difference.

1. Keep a symptom journal

Record new symptoms, when they begin, how severe they are, how long they last, and whether they affect sleep, eating, mobility, or daily responsibilities. Include changes in bowel habits, breathing, mood, and energy. Share this information with the oncology team.

2. Report symptoms early

Do not wait for the next scheduled infusion or appointment to report new or worsening symptoms. Immune-related complications are often easier to manage when recognized early.

3. Protect energy, hydration, and nutrition

Rest when needed, spread tasks throughout the day, drink fluids as medically appropriate, and seek help if eating or drinking becomes difficult. Ask the care team about dietary changes appropriate for specific symptoms.

4. Accept practical and emotional support

Ask trusted people to help with meals, transportation, household tasks, or appointments. Consider professional counseling or a cancer support group if stress, fear, or isolation becomes difficult to manage.

5. Keep scheduled laboratory tests and follow-up appointments

Blood tests and clinical assessments can help detect problems involving the thyroid, liver, kidneys, blood sugar, and other systems. Monitoring is important even when a patient feels well.

Patients should not start supplements, herbal products, steroids, or other medications to manage symptoms without discussing them with their healthcare team. Some products can interact with treatment or complicate the evaluation of a new symptom.

If an immune-related side effect develops, clinicians may pause or discontinue Keytruda and prescribe corticosteroids or other immunosuppressive treatments, depending on the severity and the organ involved. Some endocrine problems instead require hormone replacement. Treatment decisions are individualized.

When Should a Patient Seek Medical Attention?

Knowing when to call the oncology team is an important part of receiving immunotherapy safely.

Contact the oncology team promptly about new or worsening diarrhea, persistent abdominal pain, unusual fatigue, a new cough, shortness of breath, unexplained skin changes, changes in urination, or symptoms suggesting a hormone problem.

Seek emergency medical care for symptoms such as:

  • Severe or sudden difficulty breathing.
  • Chest pain, fainting, or a new severe irregular heartbeat.
  • Confusion, sudden severe weakness, or difficulty swallowing.
  • Severe abdominal pain or significant bloody stools.
  • Severe weakness accompanied by vomiting, dizziness, or collapse.
  • A rapidly spreading, blistering, or peeling rash, particularly with sores affecting the mouth or eyes.

These symptoms do not necessarily mean Keytruda is the cause, but they can signal serious medical problems that need immediate assessment. Do not wait for a scheduled appointment or assume a symptom is harmless because it seems unrelated to cancer treatment.

Patients should keep their oncology team’s contact information available and tell every healthcare professional they see that they are receiving, or have recently received, immunotherapy.

Can Keytruda Side Effects Continue After Treatment Ends?

Yes. This is one of the most important things for patients and caregivers to understand.

Some side effects improve after treatment is paused or completed. Others can persist, and certain immune-related complications may first appear weeks or months after the last dose.

For example, a patient who develops an underactive thyroid may need thyroid hormone replacement long-term. Some people experience lasting changes in other hormone systems or organ function.

This can create an unexpected transition: the cancer treatment schedule ends, but medical appointments, medications, symptom monitoring, or lifestyle adjustments continue.

Patients should ask their oncology team about long-term monitoring and what symptoms to watch for after treatment. Follow-up care remains important even when treatment has been successful.

Is Keytruda Worth the Risks?

There is no single answer that applies to every patient.

For someone with a cancer that has a strong likelihood of responding to Keytruda, the possibility of prolonged cancer control or a reduced risk of recurrence may make treatment an appealing option. For another person, the expected benefit may be smaller, or existing medical conditions may make the risks more complicated.

The decision should consider the type and stage of cancer, available alternatives, the likelihood of benefit, potential side effects, other health conditions, and the patient’s personal goals.

Useful questions to ask an oncologist include:

  • What specific benefit do you expect Keytruda to provide in my situation?
  • Is this treatment intended to cure cancer, reduce the risk of recurrence, or control advanced disease?
  • What are the most relevant risks for my particular treatment plan?
  • Which symptoms should prompt an immediate call, and which require emergency care?
  • Could treatment affect my hormone levels, fertility, sexual health, or ability to work?
  • What monitoring will I need during treatment and after my final dose?
  • If I develop a serious side effect, what other treatment options might be available?

These conversations can help patients make decisions that reflect both their medical needs and their priorities for everyday life.

Hope and Honesty Can Exist Together

Immunotherapy represents a major advancement in cancer care, and Keytruda has helped many patients access treatment options that can offer meaningful, sometimes lasting benefits. Its ability to help the immune system recognize and fight cancer has changed the outlook for certain cancers.

But promising treatment does not mean effortless treatment.

Patients may experience fatigue that changes their routines, digestive problems that limit their independence, pain that interferes with sleep, or emotional challenges that affect relationships. Less commonly, they may face serious immune-related complications that require urgent care or leave lasting effects.

These experiences deserve to be acknowledged, not minimized.

When a side effect is described as rare, that may offer reassurance about its likelihood, but it does not make the experience less real for the person who develops it. And even common, less severe symptoms can have a significant impact when they continue for weeks or months.

The goal is not to frighten patients away from immunotherapy. It is to help them understand what treatment can offer, recognize potential problems, and feel empowered to speak up when something is wrong.

Patients deserve both hope for what treatment may accomplish and honest information about what it may cost them physically and emotionally. With informed decision-making, attentive monitoring, and support that addresses the whole person, patients and their healthcare teams can work together toward the best possible outcome.

Educational resources

For additional information about Keytruda and immunotherapy, consult these reliable sources:

Citations.

National Cancer Institute

DailyMed

NIH

NIA

CDC

Merck & Co.