Disease-Modifying Therapy — MS (Interferon) Pregnancy: A large amount of data (more than 1000 pregnancy outcomes) from registries and post-marketing experience indicates no increased risk of major congenital anomalies after pre-conception exposure or exposure during the first trimester; experience with exposure in the second and third trimesters is very limited. Animal data suggest a possibly increased risk of spontaneous abortion, although the available human data do not suggest an increased risk so far. If clinically needed, use of Avonex may be considered during pregnancy. Levels of interferon beta-1a excreted in human milk are expected to be negligible and Avonex can be used during breast-feeding.

Interferon Beta-1a

Brand names: Avonex, Rebif

Interferon beta-1a is a recombinant immunomodulatory protein used as a disease-modifying therapy for relapsing-remitting multiple sclerosis.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: 30 micrograms (0.5 ml solution)

Route: Intramuscular (IM) injection — the injection site should be varied each week; a 25 mm, 25 gauge needle may be prescribed for patients for whom such a needle is appropriate

Frequency: Once a week

Max: No additional benefit has been shown by administering a higher dose (60 micrograms) once a week

UK SPC §4.2 (AVONEX 30 micrograms/0.5 ml solution for injection): 'The recommended dosage for the treatment of relapsing MS is 30 micrograms (0.5 ml solution), administered by intramuscular (IM) injection once a week.' Treatment should be initiated under supervision of a physician experienced in the treatment of the disease. PRODUCT-SPECIFICITY: this dose is for the AVONEX intramuscular once-weekly presentation of interferon beta-1a; other interferon beta-1a products and presentations may have different strengths, routes and schedules — confirm against the SPC of the specific product being prescribed. TITRATION (to reduce the incidence and severity of flu-like symptoms): therapy may be initiated at 1/4 dose increments per week reaching the full dose (30 micrograms/week) by the fourth week, or alternatively at approximately 1/2 dose once a week before increasing to the full dose; the AVOSTARTCLIP titration kit is designed for use with the pre-filled syringe only and can be used to achieve the 1/4 or 1/2 dose increments, each clip used once then discarded with any remaining product. A dose of 30 micrograms once a week should be reached and maintained after the initial titration period. An antipyretic analgesic is advised prior to injection and for an additional 24 hours after each injection to decrease flu-like symptoms, which are usually present during the first few months of treatment. DURATION: it is not known for how long patients should be treated; patients should be clinically evaluated after two years of treatment and longer-term treatment decided on an individual basis. Treatment should be discontinued if the patient develops chronic progressive MS. ELDERLY: clinical studies did not include sufficient patients aged 65 and over, but there are no theoretical reasons for dose adjustment based on the mode of clearance. PAEDIATRIC: safety and efficacy in children and adolescents aged 10 to 18 years have not yet been fully established and NO POSOLOGY RECOMMENDATION CAN BE MADE; safety and efficacy below 10 years of age have not been established (no data available). Verify any paediatric use against a children's formulary.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

History of hypersensitivity to natural or recombinant interferon beta or to any of the excipients
Patients with current severe depression and/or suicidal ideation

Side effects

Flu-like symptoms — the highest incidence of adverse reactions; most commonly myalgia, fever, chills, sweating, asthenia, headache and nausea
Headache — very common
Muscle spasticity and hypoesthesia; vomiting, diarrhoea, nausea; rhinorrhoea — common
Laboratory abnormalities: lymphocyte, white blood cell and neutrophil count decreased, haematocrit decreased, blood potassium and blood urea nitrogen increased — common
Transient neurological symptoms that may mimic MS exacerbations, including transient episodes of hypertonia and/or severe muscular weakness after injections; thrombotic microangiopathy (TTP/HUS), nephrotic syndrome, pulmonary arterial hypertension, cardiomyopathy, congestive heart failure and seizures (rare/not known)

Clinical monograph

How it works

It modulates the immune response, reducing pro-inflammatory cytokine activity and lymphocyte migration into the central nervous system, which lowers inflammatory demyelination.

Prescribing in practice

Monitor for hepatic injury, as raised transaminases and rare serious liver dysfunction can occur, and check liver function before and during treatment.
Flu-like symptoms are common early in therapy and can be reduced with gradual titration and antipyretic cover.
Use with caution in patients with depression, seizure disorders or thyroid dysfunction.

Monitoring

Monitor full blood count, liver function and thyroid function tests periodically during treatment.

Counselling the patient

Rotate injection sites and watch for skin reactions or signs of injection-site necrosis.
Expect flu-like symptoms early on that usually settle with time.
Report low mood, jaundice or unexplained bleeding to your team.

Evidence & guidelines

Interferon beta-1a is supported by pivotal trials demonstrating reduced relapse frequency and disability progression in relapsing-remitting multiple sclerosis.

Reference: NICE TA32/TA127 (Interferons for MS); PRISMS Trial; MSCRG Trial; Confirm identity and dosing against the manufacturer SPC (eMC) and NICE. Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways