Plegridy (peginterferon beta 1a)

Plegridy (peginterferon beta 1a) is a disease modifying drug (DMD) for relapsing remitting MS.

You self-inject Plegridy under the skin once every two weeks to reduce the number and severity of relapses. It reduces the number of relapses by about one third (30%).

Common side effects include flu-like symptoms and injection site reactions.

What is Plegridy used for in MS?

Plegridy is a disease modifying drug for relapsing remitting MS. You have fewer relapses than you might have had with no treatment and any relapses you do have should be less severe.

Plegridy is a moderately effective (category 1.1) DMD; in clinical trials people taking Plegridy had about 30% fewer relapses than people taking placebo. In clinical trials, MRI scans showed people taking Plegridy had fewer, smaller or no new areas of active MS (lesions). Plegridy may also slow down the build-up of disability associated with MS.

Who can take Plegridy?

Plegridy can be prescribed for adults with active relapsing remitting MS.

Plegridy has been approved for use on the NHS since 2015. It can only be prescribed by a neurologist.


It's important that you tell your MS team if you have any health problems or are taking other medicines. Plegridy may not be suitable if you have severe depression or suicidal thoughts.

Conception and pregnancy

The prescribing advice for Plegridy during pregnancy and breastfeeding is appropriately cautious. However, pregnancy registers and clinical experience have shown no increased risk to the health of mother or baby with Plegridy.

If you are trying for a family, talk to your MS nurse or neurologist about whether you should continue to take Plegridy until you are pregnant.

If you become pregnant while on Plegridy, your neurologist or MS nurse may recommend you stop taking it, depending on how active your MS has been and the benefits and risks to you and your baby. Women usually experience fewer relapses during pregnancy, so continuing to take Plegridy may not be necessary. However, if Plegridy is stopped during pregnancy, it will take several months to reach full effectiveness when restarted and may not reduce relapses during the first few months after childbirth.

Pregnancy guidelines consider that it is safe to take Plegridy during breastfeeding if required.

How do I take Plegridy?

You self-inject Plegridy under the skin once every two weeks. Plegridy is supplied in single-use, pre-filled injection pens.

Your MS nurse will show you how to do the injections, discuss the practicalities and offer advice or training and ongoing support if you should need it.

To give your body a chance to get used to Plegridy and reduce the impact of side effects, you start on low doses for the first two injections, increasing to the full dose with the third injection.

MS specialist nurse, Debbie McMillan, demonstrates how to inject Plegridy using an auto-injection device

What side effects could I get with Plegridy?


Common side effects include:

  • Flu-like symptoms, such as headache, muscle ache and stiffness, chills or fever, following an injection
  • Injection site reactions

You are more likely to have these side effects when you first start taking Plegridy. Most people have mild to moderate side effects which tend to go away over time.

A neurologist or MS specialist nurse may suggest ways to reduce these side effects including:

  • To help with flu-like symptoms, it may be helpful to change the time of day of injection in order to sleep through the worst of the side effects
  • To help reduce fever, paracetamol or ibuprofen can be taken before the injection and at four to six hour intervals after the injection, as required

Common side effects (affecting more than 1 person in 100)

  • flu-like symptoms
  • headache
  • injection site reactions
  • blood cell abnormalities
  • feeling weak or tired
  • difficulty sleeping
  • diarrhoea, nausea and vomiting
  • muscular or joint pain
  • infections

Less common side effects (affecting less than 1 person in 100)

  • changes in menstruation (periods)
  • neurological symptoms, mood changes, depression
  • liver abnormalities
  • allergic reactions
  • heart problems and hypertension
  • damage to small blood vessels leading to kidney problems

A full list of side effects is included in the manufacturer's Patient Information Leaflet.

Assessment before treatment

Before starting Plegridy, you should have blood tests to measure blood cell counts and check liver function.

Assessment during treatment

Once you've started treatment, you'll have blood tests to measure blood cell counts and monitor liver function, generally every three months for the first year, then less frequently. Depending on local practice, the tests may be carried out at a local GP surgery or it may be necessary to attend a hospital clinic. You may also have tests of your thyroid function.

During the coronavirus outbreak, you may find that your regular blood tests happen less frequently, may take place in a different location or may temporarily stop. The Association of British Neurologists has assessed the risks and benefits of blood monitoring for people taking DMDs, and has recommended a safe minimum schedule during this period. The recommendation for Plegridy is that the first blood test, 3 months after starting to take the drug, is essential, but that further tests can be halted temporarily. If you notice any new or worsening MS symptoms, you should contact your MS team.

How does Plegridy work?

Interferons are proteins that occur naturally in the immune system. It is thought that beta interferon acts by reducing both inflammation and the immune response that is attacking the body's own myelin. Plegridy is formed by adding polyethylene glycol to interferon beta 1a, which maintains the effect of the drug in the body for longer.

Plegridy research

One large study has provided evidence to support the approval of Plegridy for multiple sclerosis:

  • ADVANCE - 2014 - Plegridy compared to placebo

This two year study compared Plegridy taken by injection once every two weeks or once every four weeks with placebo in 1,512 people. At the end of the first year, Plegridy reduced the relapse rate by 35.6% in the two weekly dosing group compared to placebo. Plegridy also reduced the number of brain lesions on MRI and the risk of worsening disability.

Find out more

Scottish Medicines Consortium
Advice: peginterferon beta 1a (Plegridy)
Full guideline (link is external)
All Wales Medicines Strategy Group
Appraisal information: peginterferon beta 1a (Plegridy)
Full guideline (link is external)
Calabresi PA, et al.
Pegylated interferon β-1a for relapsing-remitting multiple sclerosis (ADVANCE): a randomised, phase 3, double-blind study.
Lancet Neurology 2014;13:657-65.
Summary (link is external)
Kieseier BC, et al.
Peginterferon beta-1a in multiple sclerosis: 2-year results from ADVANCE.
Multiple Sclerosis 2015;21(8): 1025–1035.
Full article (link is external)
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