Methotrexate and penicillin-type antibiotics such as ampicillin and amoxicillin

The NHS explains that methotrexate calms the immune system. That is the point of the treatment in rheumatoid arthritis, psoriasis and Crohn's disease, but a quieter immune system also means that infections may be easier to catch. The NHS common questions page says methotrexate may lower the number of white cells in the blood, makes infections more likely, and advises avoiding close contact with people who are known to be unwell when possible.

So it is not surprising that someone taking methotrexate may need an antibiotic for a chest infection, a urinary infection, a dental abscess or a skin infection. Ampicillin is a penicillin-type antibiotic, and so is amoxicillin, which is the one most often used in community care and the one for which the NHS publishes a patient page. The practical question is how to make sure that the person prescribing the antibiotic and the team that prescribes methotrexate are aware of each other.

Two pages address the pairing from opposite directions. The page on taking methotrexate with other medicines says that many medicines affect the way methotrexate works, and lists co-trimoxazole, trimethoprim or other antibiotics for bacterial infections among those to mention to your doctor before starting treatment.

The amoxicillin page says that some medicines affect the way amoxicillin works and increase the chance of side effects, and asks you to check with a pharmacist or doctor if you take any other medicines including methotrexate, warfarin, the gout medicines probenecid or allopurinol, or other antibiotics.

Neither page spells out the mechanism or gives figures, and this guide does not add any. What the pages establish is that a clinician should know about both medicines at the same time. How they handle it, whether by choosing a different antibiotic, adjusting timing, or arranging extra blood tests, is a decision for them.

The NHS asks people on methotrexate to tell their doctor if they think they are getting a sore throat or have a high temperature. On the side-effects page, a high temperature, or feeling hot, cold or shivery, muscle aches and a sore throat are listed among the signs of infection that should lead to telling your doctor or calling 111 immediately. Persistent cough, chest pain or breathlessness can be a sign of lung inflammation, and the NHS lists those too.

In practice, that means you should not wait to see whether a fever settles, and you should not simply take left-over antibiotics from an earlier illness. If you contact a GP, out-of-hours service or NHS 111, say at once that you take methotrexate and give your weekly dose. If you are seen by a clinician who is not part of your usual team, show the patient card.

• Name the medicine, the dose and the day of the week you take it.

• Mention any recent blood test results and when the next one is due, if you know.

• Say whether you also take folic acid, which the NHS says is often prescribed with methotrexate.

• Ask whether any adjustment to methotrexate is needed while you take the antibiotic and who should be told.

The NHS says you may be prescribed folic acid tablets to take with methotrexate to reduce some side effects such as sickness and diarrhoea, usually as a 5mg tablet once a week taken the day after methotrexate. It warns not to take folic acid on the same day as methotrexate because that can stop the medicine working properly, and not to take extra vitamin products containing folic acid if you are already taking prescribed folic acid.

Antibiotics can upset the stomach and cause diarrhoea. The NHS notes that if you take the combined or progestogen-only pill and are sick or have severe diarrhoea for more than 24 hours your contraception may be affected, and advises drinking plenty of fluids to avoid dehydration. Mention any new sickness or diarrhoea to your team, because methotrexate has its own gut side effects and it is easy to blame the wrong medicine.

The NHS says people starting methotrexate have blood, liver and kidney tests every 1 to 2 weeks and, once stable, every 2 to 3 months. Those tests are the safety net. A course of antibiotics, particularly if it is prolonged or comes with an illness that causes poor eating, vomiting or dehydration, is a reasonable moment to ask whether a blood test should be brought forward.

In the NHS, the usual route is to phone the specialist nurse line or your GP. Community pharmacists can check the antibiotic against your medicines. If you obtain a prescription from a private provider or dentist, they will not automatically see your NHS record, so volunteer the information.