Pimozide and clarithromycin: why the BNF says to avoid the combination

Pimozide is an antipsychotic. In the BNF, the licensed uses listed are schizophrenia, and monosymptomatic hypochondriacal psychosis and paranoid psychosis. It is prescribed and monitored by specialist mental health teams, and it is far less commonly used than many newer antipsychotics.

Clarithromycin is a macrolide antibiotic. The NHS describes it as a medicine to treat bacterial infections such as chest and ear infections, and lists related conditions including cellulitis, chest infection, ear infections and stomach ulcer. It is one of the antibiotics a GP, out-of-hours doctor or hospital team might choose, which is why the question of what else a person takes matters at the moment it is prescribed.

People who take pimozide often see several clinicians, so an antibiotic prescribed for a short illness can be started by someone who does not see the full list of medicines. That is the practical setting in which this interaction becomes relevant.

The BNF is the formulary used by UK prescribers and pharmacists. Its interaction entry for clarithromycin states that clarithromycin is predicted to increase the exposure to pimozide. It records the manufacturer advice as avoid, and the severity as severe, with the evidence type given as study.

In the BNF scheme a severe rating means the interaction is one prescribers are expected to take seriously, and the wording avoid means the combination should not routinely be used. It does not mean harm is certain in every person. It means that the combination is judged to carry a risk that is not worth taking when other options exist.

• Predicted to increase exposure: the antibiotic is expected to raise how much pimozide is circulating.

• Manufacturer advises avoid: the product information tells prescribers not to combine them.

• Severity severe: the BNF marks this among its more serious interaction categories.

The BNF monograph for pimozide gives some clues about why levels matter. Its contra-indications include CNS depression, comatose states, a history of arrhythmias, a history or family history of congenital QT prolongation, and phaeochromocytoma. Under cautions it notes that an ECG may be required, particularly where there are cardiovascular risk factors or a personal history of cardiovascular disease.

Put simply, pimozide is a medicine for which heart rhythm and the QT interval are part of the safety picture. A combination that pushes up the amount of pimozide in the body can move a person away from the dose that was chosen for them. This site has a separate general guide to medicine-induced QT prolongation, which explains the term for readers who want more background.

The NHS clarithromycin medicine page says there are some medicines that do not mix well with clarithromycin and advises checking with a pharmacist or doctor if you take other medicines. It lists several examples, such as epilepsy medicines, theophylline, colchicine, digoxin, warfarin, statins and domperidone. Pimozide is not in that short NHS list, which says it is not a complete list, so the BNF is the more specific source for this pair.

Practical steps are simple. Say that you take pimozide when you are prescribed any antibiotic. Ask the pharmacist to check the combination before you collect it. If an antibiotic has already been started and you only then realise, contact the prescriber, your pharmacist or NHS 111 the same day rather than waiting.

Do not stop pimozide on your own to make way for an antibiotic. Stopping an antipsychotic suddenly can cause its own problems, and the decision belongs with the prescriber. The usual solution is for the infection prescriber to choose a different antibiotic, which is a routine request.

There is no single symptom that proves this interaction is causing harm, and the NHS pages read for this guide do not give a pimozide-specific list. As general safety advice, new palpitations, a racing or irregular heartbeat, dizziness, fainting or collapse in someone taking pimozide with another medicine should be treated as urgent. Call NHS 111 or your prescriber the same day for milder concerns, and call 999 for collapse, chest pain or loss of consciousness.

Keep an up-to-date medicine list, including over-the-counter and herbal products, and take it to every appointment. The NHS pages repeatedly advise telling a doctor or pharmacist about any other medicines, herbal remedies, vitamins or supplements.

Within the NHS, the GP practice, the community mental health team and the community pharmacist can all check medicines, and hospital pharmacists review prescriptions on admission. Many pharmacies also offer structured reviews for people who have started a new medicine.

Private prescribers and private pharmacies use the same BNF and the same product information. The safety check is only as good as the medicine list that they are given, so the same habit applies: say that you take pimozide each time.