CYP2C19: clopidogrel, omeprazole and other medicines the enzyme handles

MedlinePlus Genetics says the CYP2C19 gene produces an enzyme in liver cells that processes around ten per cent of commonly prescribed medicines. The list includes clopidogrel, proton pump inhibitors for stomach acid, some antidepressants, some anticonvulsants, certain sedatives and sleeping medicines, and some antimalarial and antiretroviral medicines.

For some of these the enzyme clears the drug, and for others it turns an inactive form into an active one. That difference decides which direction a genetic variant pushes the effect, and it is why one gene can produce both too little and too much effect in different medicines.

Clopidogrel is prescribed to stop platelets clumping, for example after a heart attack, after a stent or after certain strokes. It is a prodrug, so CYP2C19 has to convert it into the form that blocks platelets. If the enzyme is weak, less active drug forms.

MedlinePlus Genetics describes poor metabolisers as carrying two non-functioning copies, such as the *2 or *3 variants, and says people with reduced function may experience treatment failure with clopidogrel, which raises the chance of heart attack or stroke. The FDA table of pharmacogenetic associations lists this relationship, although what a clinician does with the result depends on the clinical setting.

• Normal metabolisers: the usual conversion is expected.

• Intermediate metabolisers: one working and one non-working copy, so conversion is reduced.

• Poor metabolisers: little or no activation, so another antiplatelet medicine may be considered.

• Ultra-rapid metabolisers: increased enzyme activity, linked to the CYP2C19*17 variant.

People on clopidogrel are often also given a stomach-protecting medicine, because antiplatelets can irritate the gut lining. The MHRA explains that omeprazole competitively inhibits CYP2C19, which is the enzyme clopidogrel needs, so platelet blocking can fall.

The agency's updated advice discourages concurrent omeprazole or esomeprazole unless essential, and recommends considering another PPI, an H2 blocker other than cimetidine, or an antacid. It also says prescribers should check whether a patient is buying omeprazole over the counter. Importantly the MHRA noted that large clinical trials had not shown clear cardiovascular harm from PPI use overall, which is why the advice is limited to the two named drugs rather than the whole class.

The NHS clopidogrel page confirms that omeprazole and esomeprazole may affect how well clopidogrel works, and that a doctor may swap in an alternative such as lansoprazole. The NHS omeprazole page adds that over-the-counter omeprazole should not be taken for longer than two weeks without advice and warns anyone on blood thinners to check first.

The same NHS page on clopidogrel interactions mentions SSRIs such as citalopram, rosuvastatin, rifampicin, carbamazepine and St John's wort, so a medicines review is worthwhile whenever a new product is added. Pharmacists can check a full list quickly, and they are the sensible first stop before buying any indigestion or herbal product.

Routine genotyping before clopidogrel is not a feature of every UK service, and practice varies by hospital and by indication. Where a test is used, it is typically in settings where the stakes are high, such as after a stent, and the result guides the choice of antiplatelet rather than a dose.

If you are told you are a poor metaboliser, do not stop clopidogrel yourself. Stopping an antiplatelet suddenly after a stent or heart event can be dangerous, and any change should come from your cardiology team.

Because the enzyme also handles some antidepressants and anticonvulsants, a result can occasionally explain unusual side effects or poor response with those drugs. Interpretation is specific to each medicine and is done by a pharmacist or specialist.

Seek urgent help (999) for chest pain, stroke symptoms or heavy bleeding. For black stools, vomiting blood or severe unexplained bruising while on clopidogrel, contact NHS 111 or your doctor promptly.