Keytruda (pembrolizumab) is a brand-name solution for IV infusion prescribed to treat many types of cancer, including melanoma and lung cancer. As with other drugs, Keytruda can cause side effects, such as eye-related problems, skin reactions, and hair loss.

Keytruda is a biologic that belongs to the programmed cell death protein 1 (PD-1) inhibitor drug class. Read on to learn about potential common, mild, and serious side effects of Keytruda. For more information on Keytruda, including a list of conditions it treats, see this Keytruda overview article.

Keytruda can cause certain side effects, some of which are more common than others. These side effects may be temporary, lasting a few days or weeks. But if the side effects last longer than that, bother you, or become severe, be sure to talk with your doctor or pharmacist.

These are just a few of the more common side effects reported by people who took Keytruda in clinical trials:

It’s important to note that these side effects can vary depending on whether you are using Keytruda in combination with other drugs to treat your condition. Also note that the side effects of Keytruda aren’t known to vary depending on the type of cancer it’s treating, such as lung cancer.

* For more information on this side effect, see the “Side effect specifics” section later in the article.

Mild side effects can occur with Keytruda use. This list doesn’t include all possible mild side effects of the drug. For more information, you can refer to Keytruda’s prescribing information.

Mild side effects that people receiving Keytruda have reported in clinical trials include:

These side effects may be temporary, lasting a few days or weeks. But if the side effects last longer than that, bother you, or become severe, be sure to talk with your doctor or pharmacist.

Note: After the Food and Drug Administration (FDA) approves a drug, it tracks side effects of the medication. If you develop a side effect while receiving Keytruda and want to tell the FDA about it, visit MedWatch.

* For more information on this side effect, see the “Side effect specifics” section later in this article.

Keytruda may cause serious side effects. The list that follows may not include all possible serious side effects of the drug. For more information, you can refer to Keytruda’s prescribing information.

If you develop serious side effects while receiving Keytruda, call your doctor right away. If the side effects seem life threatening or you think you’re having a medical emergency, immediately call 911 or your local emergency number.

Serious side effects and their symptoms that people receiving Keytruda have reported in clinical trials include:

  • Severe infusion reactions (side effects that occur during or shortly after receiving a Keytruda infusion). Symptoms can include:
  • Severe immune system reactions involving inflammation. Symptoms can vary widely depending on which area of the body is affected and can include:
  • Eye-related side effects.*
  • Severe skin reactions.*
  • Confusion.*
  • Allergic reaction.*

* For more information on this side effect, see the “Side effect specifics” section later in this article.

Long-term side effects

When you’re considering Keytruda treatment, you may want to know how long Keytruda’s side effects last.

The more common and mild side effects of Keytruda are usually temporary. Some may go away once your body gets used to the drug, within a few days or weeks. Others may continue for as long as you’re receiving Keytruda treatment.

However, some serious side effects of Keytruda could cause long-term effects. Although less common, severe immune system reactions can happen with Keytruda. These can occur because Keytruda may trigger your immune system to attack your body and cause inflammation. Depending on which area of your body is affected, the inflammation could cause long-term damage.

If you have questions about long-term side effects of Keytruda, talk with your doctor.

Keytruda can be used to treat skin cancer, lymphoma, and some types of solid cancer tumors in certain children. In clinical trials, certain side effects of Keytruda were reported more often in children than in adults. These include:

Learn more details about some of the side effects that Keytruda may cause.

Eye-related side effects

Some people receiving Keytruda have experienced ocular (eye-related) side effects during clinical trials. Some examples of these eye problems include:

  • Uveitis (inflammation of the uvea, which is the middle layer of your eye). This can cause eye pain, watering, and itchiness.
  • Iritis (inflammation of the iris, which is the colored part of your eye surrounding the pupil). This can cause pain, redness, sensitivity to light, and decreased vision.
  • Retinal detachment (a condition in which the back inner lining of your eye starts to separate from the rest of your eye). This can cause sudden vision loss.

However, eye-related side effects were rare during clinical trials of Keytruda.

What you can do

If you notice any eye or vision problems after starting Keytruda, do not wait to see if your symptoms get better. It’s important to call your doctor right away or get emergency medical care. You may receive an immediate referral to an ophthalmologist (a doctor who specializes in the eye).

If you get immediate treatment for possible eye problems (which may include surgery in severe cases), your eyes will have a better chance of recovering. Your doctor may have you stop Keytruda treatment if you have eye-related side effects.

Even if you’re not having vision problems, it’s important to visit your eye doctor regularly while you’re receiving Keytruda. They can monitor you for any vision changes.

Skin reactions

Serious skin reactions, such as certain types of severe rashes, have occurred in clinical trials of Keytruda. These reactions included Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN). Both of these conditions cause symptoms such as:

  • severe, painful blisters on the body
  • skin peeling off in layers
  • flu-like symptoms, such as fever, chills, and body aches

SJS and TEN can be life threatening.

Note: A mild, itchy skin rash is a common side effect of Keytruda. But the serious skin reactions just described are different from this mild side effect and require immediate medical care.

What you can do

If you develop a skin reaction that involves blisters or any symptoms that seem severe, talk with your doctor right away. They may recommend that you call 911 or seek emergency medical care.

If you have a severe skin reaction, your doctor will likely have you permanently stop Keytruda treatment.

Pain in muscles or bones

Bone pain and muscle pain are common side effects of Keytruda.

Some people who have experienced bone pain describe it as a deep, dull ache in one or more bones. The pain is usually present whether you’re moving or at rest. You may also notice pain, stiffness, or swelling in your joints.

With muscle pain, your muscles may feel very sore, especially with movement.

The severity of muscle or bone pain from Keytruda can vary. If you already have conditions that affect your muscles or bones, such as arthritis, these side effects can be more severe.

What you can do

Muscle or bone pain from Keytruda tends to be mild and easily manageable in most cases. The following home remedies may help:

  • taking an over-the-counter pain reliever, such as Tylenol (acetaminophen), as long as your doctor recommends it
  • heat therapy and cold therapy
  • “listening” to your body and resting when you need to

If these remedies aren’t helpful, or if you have pain that becomes severe, talk with your doctor. They may recommend a prescription pain medication. They may also suggest seeing an orthopedic (bone) specialist.

Confusion

Confusion can occur when receiving Keytruda treatment. Although it may seem like a mild side effect, confusion could signify something more serious.

Keytruda can cause severe immune system reactions in which your immune system attacks your body. This can cause inflammation, which can lead to varied symptoms depending on the area of the body that’s affected. It’s possible that confusion could be a sign you’re developing an immune system reaction from Keytruda.

Confusion could occur if inflammation is building up in certain areas of your body, such as:

  • your nervous system (brain and spinal cord)
  • your adrenal glands (glands located on top of your kidneys that make a hormone called adrenaline)

What you can do

Consider letting your loved ones (or a caregiver if you have one) know that you may experience confusion during treatment. If you or those around you notice any new or unusual confusion after you start Keytruda treatment, do not wait to see whether this gets better on its own. It’s important to talk with your doctor about this right away.

Your doctor may recommend testing you for an immune system reaction. Depending on this evaluation, you may need treatment in a hospital. Your doctor will likely have you stop Keytruda treatment if you have an immune system reaction.

Hair loss

In clinical trials that looked at Keytruda in combination with chemotherapy, hair loss was one of the most common side effects. Hair loss is a common side effect of many chemotherapy drugs. Hair loss was not seen when Keytruda was used on its own.

Keytruda can cause severe immune system reactions in which your immune system attacks your body. This can cause inflammation, which can lead to varied symptoms depending on the area of the body that’s affected. Hair loss could be a sign that Keytruda is causing inflammation of your thyroid gland. Your thyroid is a gland in your neck that produces certain hormones.

Other symptoms of thyroid gland inflammation can include weight loss or gain, increased sweating or feeling cold, mood changes, and heart rate changes. Diarrhea and constipation are also possible symptoms.

What you can do

Hair loss is a very common side effect of many cancer treatments. But this side effect usually goes away when your treatment ends. After you’ve completed treatment, your hair will start to grow back.

However, you should always tell your doctor if you’re losing hair. This is especially important if you’re receiving Keytruda on its own (without chemotherapy) or if you’re having other symptoms of thyroid inflammation.

Your doctor may suggest a blood test to check your thyroid hormone levels. They’ll also check for other signs of an immune system reaction that could be affecting your thyroid. Depending on what your doctor determines to be the cause of your hair loss, they may have you stop Keytruda treatment.

Allergic reaction

As with most drugs, Keytruda can cause an allergic reaction in some people.

Symptoms can be mild or serious and can include:

  • rash
  • itching
  • flushing
  • swelling under your skin, typically in your lips, eyelids, feet, or hands
  • swelling of your mouth, tongue, or throat, which can make it hard to breathe

What you can do

For mild symptoms of an allergic reaction, call your doctor right away. They may recommend ways to ease your symptoms and determine whether you should keep using Keytruda. But if your symptoms are serious and you think you’re having a medical emergency, immediately call 911 or your local emergency number.

Here are some frequently asked questions about Keytruda’s side effects and their answers.

When do Keytruda side effects start?

It can vary. Side effects from Keytruda may happen at any time.

You may experience side effects when you first receive Keytruda, which a healthcare professional will administer as an intravenous (IV) infusion. These are called infusion reactions, and they can sometimes be severe. (For more information, see the “Serious side effects of Keytruda” section earlier in this article.) A healthcare professional will monitor you for such side effects during your infusions.

Other side effects could happen weeks or months after you start receiving the drug. They may even happen after stopping Keytruda treatment.

Whether you receive Keytruda alone or in combination with other drugs can affect the timing of side effects. Certain risk factors may also affect when you might develop side effects. (For more information, see the “Precautions for Keytruda” section later in this article.)

To find out when you might experience side effects from receiving Keytruda, talk with your doctor.

Are Keytruda side effects cumulative?

It’s possible. Most side effects are short term and may be temporary, lasting a few days or weeks. And some may go away as you continue using Keytruda. However, some may worsen as you continue treatment. Examples include diarrhea, inflammation of the lungs, and increased liver enzymes. Keep in mind that you may take Keytruda with other medications, which could also worsen certain side effects.

If you have certain health conditions, such as lung problems or liver disease, Keytruda may cause them to worsen. (For more information, see the “Precautions for Keytruda” section later in this article.)

Before starting treatment, be sure to tell your doctor about any health conditions you have. This helps your doctor determine whether to monitor you more closely for side effects you might have a higher risk of. Contact your doctor if you develop any side effect symptoms. Early treatment of side effects may help prevent them from worsening. Your doctor can let you know what symptoms to watch for during treatment.

What percentage of people taking Keytruda have side effects?

Like any drug, Keytruda can cause side effects. However, the percentage of people who develop side effects from Keytruda treatment is unknown.

Clinical trials report on side effects experienced by people who participated in them. However, the trials have varying conditions. (For example, some clinical trials of Keytruda looked at the use of the drug with other medications.) As a result, rates of side effects can differ between trials.

Side effect rates in clinical trials may not accurately reflect side effect rates after the drug becomes available. Whether a person has side effects from Keytruda will vary from person to person.

If you have concerns about developing side effects from Keytruda, talk with your doctor. They can discuss what other people have generally experienced while receiving this drug.

Be sure to talk with your doctor about your health history before you start Keytruda. This drug may not be the right treatment for you if you have certain medical conditions or other factors that affect your health. The conditions and factors to consider include:

Prior stem cell transplant: If you’ve received an allogeneic stem cell transplant (stem cells from a donor), Keytruda may increase your risk of graft-versus-host disease (GVHD). With GVHD, your immune system attacks the transplanted stem cells. If you’ve had a stem cell transplant in the past, talk with your doctor about this risk before starting Keytruda.

Use of certain drugs for multiple myeloma: Receiving Keytruda along with certain drugs that treat multiple myeloma may cause harm. These include thalidomide-like drugs taken with dexamethasone. The combination of these drugs with Keytruda has been linked to an increased risk of death. If you’re receiving these treatments for multiple myeloma, your doctor will talk with you about other cancer treatments that would be safer for you.

Allergic reaction: You should not receive Keytruda if you’ve ever had an allergic reaction to the drug or any of its ingredients. Talk with your doctor about which other treatments are better options for you.

Pregnancy: Keytruda can cause harmful effects in a developing fetus. The manufacturer recommends you use an effective form of birth control during Keytruda treatment and for 4 months after your Keytruda treatment ends. For more details, see the “Pregnancy and breastfeeding while receiving Keytruda” section later in this article.

Breastfeeding: You should not receive Keytruda while breastfeeding. For details, see the “Pregnancy and breastfeeding while receiving Keytruda” section later in this article.

Alcohol use with Keytruda

Alcohol is not known to cause any interactions with Keytruda.

But drinking alcohol, especially in large amounts, could cause some of the same possible side effects of Keytruda. These may include:

If you drink alcohol, talk with your doctor about whether any amount of alcohol is safe for you during Keytruda treatment.

Pregnancy and breastfeeding while receiving Keytruda

It’s not safe to receive Keytruda during pregnancy. The drug could cause harmful effects in a developing fetus. For this reason, it’s recommended to use an effective form of birth control during Keytruda treatment. You should continue using birth control until at least 4 months after your last dose of Keytruda. This is because the drug stays in your system for a few months after you stop treatment.

If you’re sexually active and you or your partner can become pregnant, talk with your doctor about your birth control needs while you’re using Keytruda.

You should not receive Keytruda while breastfeeding. The drug could cause harm to a child who’s breastfed by a person receiving Keytruda. And it’s recommended to wait at least 4 months after your last dose of Keytruda before breastfeeding.

Before starting Keytruda, talk with your doctor about any concerns you may have about pregnancy or breastfeeding.

Like other drugs that treat cancer, Keytruda comes with the potential for mild and serious side effects. If you’d like to learn more about Keytruda, talk with your doctor or pharmacist. They can help answer any questions you have about the drug’s side effects.

Besides talking with your doctor, you can do some research on your own. These articles might help:

Disclaimer: Medical News Today has made every effort to make certain that all information is factually correct, comprehensive, and up to date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or another healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.