Methotrexate: once-weekly dosing, monitoring and safety

The NHS describes methotrexate as an immunosuppressant that slows the immune system and helps reduce inflammation. It is used for rheumatoid arthritis, psoriasis (including psoriatic arthritis) and Crohn disease, and can also be prescribed for cancer. It calms an immune system that is attacking the body's own cells; it is not a painkiller, though pain eases as inflammation improves. In cancer treatment it stops cancer cells reproducing.

It comes as tablets, a liquid or pre-filled injection pens and syringes, and a specialist may recommend injections if the tablets or liquid are not working.

The most important safety fact is the schedule. The NHS says methotrexate for inflammatory conditions is taken once a week, on the same day each week, and you get a patient card to record the weekly dose and day. Keep it with you and show it to any healthcare staff involved in your care.

Dose depends on the condition and blood results. For rheumatoid arthritis the usual adult starting dose is 7.5mg once a week, increased slowly up to 20mg in tablet form or 25mg by injection. For psoriasis it is 2.5mg to 10mg weekly, rising to a maximum of 30mg, and for Crohn disease 10mg to 25mg weekly for maintenance. Always follow your specialist.

• Swallow tablets whole with water, before or after food, and wash your hands afterwards. You may need up to 10 tablets to make up a dose.

• Measure the liquid with the oral syringe provided, never a kitchen spoon.

• If you forget, take it the next day or the day after. If it is more than 2 days late, contact your doctor or clinic. Never take two doses together.

• Store the weekly medicine apart from daily medicines so it is not confused with them.

At first a hospital consultant usually issues a week's supply at a time. Once the dose is right and the condition stable, the GP may take over and usually provide a month's supply. Regular blood tests are part of treatment, and you may be given a booklet to record doses and results. A chest X-ray and breathing test may be done before you start to check the lungs.

Methotrexate can lower white blood cell counts, which makes infections more likely. Avoid close contact with people who are unwell when possible. Tell your doctor urgently about a sore throat, high temperature or COVID-19, and about contact with chickenpox or shingles. Have invited vaccinations, but tell the vaccinator you take methotrexate because some live vaccines are unsuitable.

More than 1 in 100 people get loss of appetite, nausea, stomach pain or indigestion, diarrhoea, headaches, tiredness or drowsiness, or hair loss that is unlikely to be permanent. Skin can become very sensitive to sunlight, so use SPF 30 or above, cover up and avoid sun lamps and sunbeds. Folic acid can reduce sickness and diarrhoea.

Rare serious effects, in fewer than 1 in 10,000 people, include yellow skin or eyes (liver problems), persistent cough or breathlessness (lung inflammation), swelling or changed urination (kidney problems), fever, sore throat or muscle aches (infection) and bleeding gums, blood in urine or unexplained bruising (blood disorders). Tell your doctor or call 111 immediately, and call 999 for anaphylaxis. Long-term, liver or lung problems are more likely at high doses and are watched for at check-ups.

Methotrexate is not recommended in pregnancy because it can cause birth defects. If you want to get pregnant, speak to your doctor before stopping contraception because you may need to switch medicine. If you become pregnant while taking it, do not stop it yourself; speak to your doctor as soon as possible.

For breastfeeding, tiny amounts pass into milk and doctors advise individually. At weekly doses of 25mg or less it may be possible, but you must not breastfeed for 24 hours after taking it, and your baby will have blood tests. It may reduce fertility, damaging sperm in men and egg production in women.

Many medicines interact. The NHS lists NSAIDs such as ibuprofen (and cough and cold remedies containing them), co-trimoxazole, trimethoprim and other antibiotics, diuretics such as indapamide, epilepsy medicines such as phenytoin or levetiracetam, theophylline, omeprazole and folic acid supplements. If you are prescribed folic acid, do not also take supplements containing it. Children must not have the live children's flu vaccine.

It is usually fine to drink alcohol on a low weekly dose of 25mg or less, but ask your doctor if the dose is higher. Avoid unpasteurised milk and soft cheeses, and avoid too much caffeine, which can stop methotrexate working as well. Alternatives for rheumatoid arthritis include sulfasalazine, biologics such as adalimumab, steroids, NSAIDs and physiotherapy.