Interferon beta-1a (Avonex) and peginterferon beta-1a (Plegridy) are both injectable treatments for relapsing-remitting multiple sclerosis (RRMS). Avonex attaches to the surfaces of your cells and helps prevent your body from attacking your central nervous system. Plegridy has the same effect on multiple sclerosis (MS) that Avonex does, but it has a longer action.


Avonex comes in a powder form that you mix yourself or in a premixed solution. The solution comes in prefilled syringes or an automatic pen injector. Avonex is injected into a major muscle once each week.


Avonex is available in a starter pack with smaller doses that allows you to increase the dose of Avonex gradually. This gradual increase in dosage helps to reduce possible flu-like side effects. The starting dose is usually 7.5 micrograms the first week. The dose increases by 7.5 micrograms every week until you reach the full dose of about 30 micrograms.


Vials of Avonex powder should be stored between 2 to 8°C (36 to 46°F) in a refrigerator. If refrigeration is not available, the vials can be stored at 25°C (77°F) for up to 30 days.

Avonex prefilled syringes should be stored between 2 to 8°C (36 to 46°F) and then warmed to room temperature for about 30 minutes before using. If you can’t refrigerate the syringe, it can be stored at no more than 25°C (77°F) for up to seven days.


Plegridy is injected subcutaneously, or under your skin. Plegridy comes in different kinds of packaging. There is a prefilled syringe and a pen-injector. There’s also a starter pack for each form, which allows you to increase the dose gradually.


Plegridy has a molecule of polyethylene, a chemical that has no drug activity, attached to each interferon beta-1a molecule. This makes the drug active in your body for a much longer time. Because of its longer action, the dose of Plegridy is 125 micrograms every two weeks.

The first dose is usually 63 micrograms, the next dose, taken two weeks later, is 95 micrograms, and the next dose, taken two weeks after the second dosage, is 125 micrograms. Gradually increasing the dose of Plegridy also helps to reduce side effects.


According to DailyMed, the drug information site maintained by the National Institute of Health (NIH) and the National Library of Medicine (NLM), Plegridy should be stored in the refrigerator, between 2 to 8°C (36 to 46°F) and warmed to room temperature before use. If refrigeration is not available, you can store Plegridy between 2 to 25°C (36 to 77°F) for up to 30 days.

The cost of using both drugs is almost exactly the same, just over $6000 per month of treatment. Most prescription insurance plans cover drugs for MS, but they may require prior authorization and you may need to work with a large specialty pharmacy that ships the drug to you. There are also assistance programs that can help you pay for your MS treatment.

FormPowder, premixed solution available in prefilled syringe or pen injectorPrefilled syringe or pen injector
Dosage30 mcg once per week125 mcg every other week
CostApproximately $6,000 per monthApproximately $6,000 per month
InsurancePrior authorization may be requiredPrior authorization may be required
AvailabilityMay need to use a large specialty pharmacyMay need to use a large specialty pharmacy
StorageRefrigerate at 2-8°C (36-46°F). If refrigeration is not available, store at 25°C (77°F) for up to 30 days (prefilled syringes can be stored at 77°F for only up to 7 days).Refrigerate at 2-8°C (36-46°F). If refrigeration is not available, store at 2-25°C (36-77°F) for up to 30 days.

Avonex and Plegridy both contain interferon beta-1a, and both have similar side effects.

Milder Side Effects

Frequent, milder side effects include:

  • headache
  • chills, fatigue, and flu-like symptoms
  • pain
  • dizziness
  • injection site reactions, including redness, swelling, pain, or even skin damage
  • stomach pain
  • low white blood cell count (found through a blood test)
  • reduced thyroid function (found through a blood test)

More Serious Side Effects

See your doctor right away if you experience any of the more serious side effects. These include:

  • depression
    • There’s a higher risk of depression if you’ve had depression before, and you could have an increased risk of suicide.
  • immune reaction
    • Both drugs contain proteins, and you could develop antibodies to them. Sometimes, there are no symptoms. It’s also possible that you could have a reaction like hives, rash, or trouble breathing. If this happens, see a doctor right away.
  • signs of liver injury
    • Signs can include increasing fatigue, yellowing of eyes or skin, or swollen or painful abdomen.
  • seizures
    • The risk is higher if you have a seizure disorder and start taking interferon beta-1a.
  • heart failure
    • If you have a history of heart disease, you have an increased risk of this rare side effect. Talk to your doctor before using interferon beta-1a.

Plegridy may have a slightly higher risk than Avonex of reducing blood counts of all kinds of blood cells.If you take either Avonex or Plegridy, your doctor will monitor your health with blood tests. If you have severe fatigue or experience increased infections, you should see your doctor right away. Infections may include:

  • injection site or other skin infections
  • sinus infection
  • bladder infection
  • vaginal infection, including yeast or bacterial infection

Many doctors will advise you to take over-the-counter pain medication or fever-reducing drugs to help reduce your risk of flu-like symptoms that commonly happen after your dose.

There hasn’t been a study that directly compares the effects of Avonex against Plegridy. The Multiple Sclerosis Coalition, though, has summarized studies about both drugs. They found that when Plegridy was compared to a placebo, those who used Plegridy had 36 percent fewer relapses after four years than those who used a placebo. When Avonex was compared to placebo, those who used Avonex had 18 percent fewer relapses per year of treatment. These outcomes represent data from this particular study, but they may not reflect your experience with either of these drugs.

Your doctor should have experience using both Avonex and Plegridy to treat people with MS, and they can answer any questions you may have about these drugs. Your personal preferences, your experience and side effects with other MS drugs, and other factors may be part of the decision. Talk to your doctor to find the treatment that will be best for you.