Simvastatin and clarithromycin: why the combination is usually avoided

Simvastatin belongs to the statin group, which the NHS describes as a medicine used to treat high cholesterol and prevent heart attacks and strokes. The NHS simvastatin page notes that if you stop taking statins your cholesterol levels may go up again, so they are not a medicine to drop casually.

Clarithromycin is an antibiotic, which the NHS says is used for infections such as chest and ear infections, and lists stomach ulcer among its related conditions.

Individually each is well established. The concern is what happens when they overlap, because the antibiotic can change how much simvastatin stays in the body.

The BNF entry for simvastatin states that clarithromycin is predicted to increase the exposure to simvastatin, with the manufacturer advising avoid, and gives it a severity of severe with evidence from studies. The BNF page for clarithromycin gives the same message from the other direction. In plain words, a given dose of simvastatin acts like a larger dose while clarithromycin is on board.

Higher statin exposure is the reason the BNF flags muscle problems. The entries for other statins show the same logic: for atorvastatin the manufacturer advises avoiding or adjusting the dose and monitoring for rhabdomyolysis, which is a serious form of muscle damage, and for pravastatin it advises caution because clarithromycin moderately increases its exposure.

The NHS pages give the practical reflection of this. The simvastatin page lists antibiotics such as fusidic acid, erythromycin and clarithromycin among medicines that may not mix well, and the clarithromycin page lists a statin such as simvastatin or atorvastatin among its cautions.

The NHS lists inflammation and damage to the muscles, causing muscle cramps, pain and weakness, among possible serious side effects of simvastatin. It also lists inflammation of the liver causing yellowing of the skin or eyes, inflammation of the pancreas causing severe stomach pain, and allergic reaction.

The NHS says to call NHS 111 if you are taking simvastatin and think you might be having serious side effects. For a person who has started clarithromycin, new unexplained muscle pain, tenderness or weakness over the next days is worth acting on promptly rather than waiting for the course to end. If symptoms are severe, with marked weakness or difficulty moving, seek urgent help through 111, and call 999 in an emergency.

Everyday muscle aches are common, and the NHS lists muscle aches among common simvastatin side effects, so not every ache is dangerous. The key is a change in pattern, especially after a new medicine has been added.

When a statin user needs an antibiotic, there are several routes a prescriber may consider. They can pick an antibiotic that does not interact in the same way, can review whether the statin should be paused for the length of a short course, or can arrange a different statin. Which is best depends on the infection, how urgent treatment is and your history. These are decisions for the prescriber and pharmacist, informed by the BNF.

The best protection is communication. Say you take simvastatin whenever an antibiotic is suggested, including by out-of-hours doctors, urgent treatment centres and pharmacists, who may not have your record. The pharmacist can check the prescription against your list before you leave.

• Mention every regular medicine, including any statin, whenever an antibiotic is prescribed.

• Ask whether the antibiotic interacts, and whether the statin should continue during the course.

• Do not stop or restart the statin by yourself without that advice.

• Report new muscle pain or weakness promptly.

Clarithromycin is not the only issue. The NHS lists antifungal medicines such as itraconazole and ketoconazole, other cholesterol medicines, erythromycin and fusidic acid, some medicines for viral infections including HIV and hepatitis C, ciclosporin, nefazodone, amlodipine, diltiazem and verapamil, and danazol. It also tells people to avoid grapefruit juice with simvastatin.

The NHS says to tell your doctor before taking simvastatin about any medicines, herbal remedies, vitamins or supplements. It lists reasons simvastatin may not suit someone, including liver or kidney problems, an underactive thyroid, unexplained ongoing muscle pain, a family history of muscle problems or regularly drinking a lot of alcohol.

In the NHS, the GP surgery usually holds your repeat prescriptions, and a dispensing pharmacist is a useful second check. Hospital teams and private clinicians use the same BNF, so the interaction advice does not change, but they may not see your repeat list. A printed list or the NHS App is a practical aid.