Plegridy (peginterferon beta-1a) is a prescription drug used to treat multiple sclerosis (MS) symptoms caused by certain conditions. Plegridy can cause side effects that range from mild to serious. Examples include injection site reactions and headache.

Specifically, Plegridy is used in adults to treat MS symptoms caused by the following conditions:

The active ingredient in Plegridy is peginterferon beta-1a. The drug comes as a liquid solution that’s given as an injection under your skin or into your muscle.

Keep reading to learn about common, mild, and serious side effects that Plegridy can cause. For a general overview of the drug, including details about its uses, see this article.

Some people may experience mild to serious side effects during Plegridy treatment. Examples of the drug’s commonly reported side effects include:

* To learn more about this side effect, see the “Side effects explained” section.

Mild side effects have been reported with Plegridy. These include:

In most cases, these side effects should be temporary. And some may be easily managed. But if you have symptoms that are ongoing or bothersome, talk with your doctor or pharmacist. And do not stop Plegridy treatment unless your doctor recommends it.

Plegridy may cause mild side effects other than those listed above. See the drug’s prescribing information for details.

* To learn more about this side effect, see the “Side effects explained” section.

Serious side effects have been reported with Plegridy. These include:

If you develop serious side effects during Plegridy treatment, 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.

* To learn more about this side effect, see the “Side effects explained” section.

Note: After the Food and Drug Administration (FDA) approves a drug, it tracks and reviews side effects of the medication. If you’d like to notify the FDA about a side effect you’ve had with Plegridy, visit MedWatch.

Allergic reaction

For some people, Plegridy may cause an allergic reaction, but this side effect was rare in the drug’s studies.

What might help

If you have mild symptoms of an allergic reaction, such as a mild rash, call your doctor right away. They may suggest a treatment to manage your symptoms and can determine whether you should continue Plegridy treatment.

If you have symptoms of a severe allergic reaction, such as swelling or difficulty breathing, call 911 or your local emergency number right away. These symptoms could be life threatening and require immediate medical care.

If your doctor confirms you’ve had a serious allergic reaction to Plegridy, they may have you switch to a different treatment.

Get answers to some frequently asked questions about Plegridy’s side effects.

How long do Plegridy side effects last? Does the drug cause long-term side effects?

How long side effects of Plegridy last can vary from person to person. Other drugs you take and medical conditions you have (such as kidney problems) can affect which side effects you experience. They can also affect how long your side effects last and how severe they are. But these are not the only factors.

Some side effects, such as flu-like symptoms and mild injection site reactions, typically last a couple of days. Others, including pulmonary arterial hypertension and liver problems (such as hepatitis) can last longer. These are typically considered long-term side effects.

Long-term side effects include those that may start at any time during treatment with a drug, even if you’ve taken it for a long time. It also includes side effects that may not go away, even after you stop treatment.

Talk with your doctor to learn more about possible side effects of Plegridy and how long they may last.

Does Plegridy cause hair loss?

It’s not likely. Hair loss wasn’t a side effect reported in the drug’s studies. But some people can experience autoimmune diseases during Plegridy treatment and these can lead to changes in thyroid hormone levels. This can cause changes in thyroid function, which can cause side effects such as hair loss.

Talk with your doctor to learn more about Plegridy, autoimmune diseases, and potential side effects.

Learn more about some of the side effects Plegridy may cause.

Injection site reactions

Mild injection site reactions were a common side effect reported in studies of Plegridy. Injection site reactions are symptoms experienced at the site where the drug was injected. Typically, these symptoms are mild and include redness (or skin discoloration), itching, and pain.

Sometimes these symptoms can be severe, such as skin necrosis (tissue death). Severe reactions can cause symptoms at the injection site such as:

  • black or blue skin discoloration
  • swelling
  • pain
  • lesions

What might help

Inject Plegridy exactly as a healthcare professional has shown or told you. The manufacturer also provides step-by-step instructions explaining how to use Plegridy. Rotating the injection site can lower your risk of site reactions.

Do not inject Plegridy in any area of the body where the skin is irritated, bruised, infected, or scarred. If multiple lesions are present, change the injection site. Your doctor may also recommend pausing Plegridy treatment until the injection site is healed.

Check the injection site about 2 hours after injecting Plegridy for symptoms of a site reaction. If symptoms don’t clear up after a few days, talk with your doctor right away.

If you experience severe injection site reaction symptoms, talk with your doctor immediately before giving yourself another dose of Plegridy.

Liver problems

Liver problems were a rare side effect reported in studies of Plegridy. These problems included changes in liver enzymes, hepatitis, and others. Liver problems can cause side effects such as:

What might help

Your doctor may have you complete blood tests regularly to check for these possible side effects. Talk with them right away if you experience any of the above symptoms. They’ll help determine whether Plegridy is still the best treatment for your condition.

Keeping track of side effects

During your Plegridy treatment, consider taking notes on any side effects you’re having. You can then share this information with your doctor. This is especially helpful when you first start a new drug or combination of treatments.

Your side effect notes can include things such as:

  • what dose of the drug you were taking when you had the side effect
  • how soon you had the side effect after starting that dose
  • what your symptoms were
  • how your symptoms affected your daily activities
  • what other medications you were taking
  • any other information you feel is important

Sharing notes with your doctor will help them learn more about how Plegridy affects you. They can then use this information to adjust your treatment plan if needed.

Plegridy can sometimes cause harmful effects in people with certain conditions. This is known as a drug-condition interaction. Other factors may also affect whether this drug is a good treatment option for you. Talk with your doctor about your health history before starting Plegridy. Be sure to tell them if any of the following factors apply to you:

  • mental health conditions, including depression
  • suicidal thoughts or behavior
  • liver problems
  • bleeding problems
  • low platelets or white blood cells
  • heart problems
  • seizure
  • thyroid problems
  • autoimmune disease
  • rubber or latex allergy
  • previous allergic reaction to Plegridy
  • pregnancy
  • breastfeeding
  • alcohol consumption

Help is out there

If you or someone you know is in crisis and considering suicide or self-harm, please seek support:

If you’re calling on behalf of someone else, stay with them until help arrives. You may remove weapons or substances that can cause harm if you can do so safely.

If you are not in the same household, stay on the phone with them until help arrives.

Disclaimer: Healthline 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.