The US Food and Drug Administration (FDA) has issued a warning to patients, asking them not to combine Plavix with Nexium, Prilosec, including 9-other drugs like Prozac and Tagamet.

According to FDA, such a combination of drugs would only make anti-clotting Plavix dangerously ineffective. The second most prescribed drug in the world, Plavix blocks an enzyme in the body responsible for turning it into its active form.

Not the first time, questions have also been raised about Prilosec, the potent stomach-acid-reducing drug that blunts the benefit of Plavix. However, data from recent studies indicate Prilosec and Nexium, a drug similar to it may actually help patients taking Plavix, leading FDA to ask drug manufacturer Sanofi-Aventis, the maker of Plavix to specifically study interactions between Plavix and Prilosec, including between Plavix and Nexium.

However, it was found that Nexium and Prilosec interfered with Plavix by inhibiting CYP 2C19, an enzyme that activates Plavix, and not because they belong to powerful class of acid-reducing drugs called proton pump inhibitors or PPIs.

FDA asks patients taking Plavix to avoid several other drugs that also inhibit CYP 2C19, such as:

" Tagamet, generically known as cimetidine.

" Prozac, Serafem, and Symbyax - generic name: fluoxetine.

" Luvox - generic name: fluvoxamine.

" Ticlid - generic name: ticlopidine.

" Diflucan - generic name: fluconazole.

" Nizoral - generic name: ketoconazole.

" VFEND - generic name: voriconazole.

" Intelence - generic name: etravirine.

" Felbatol - generic name: felbamate.

Patients taking Plavix should only stop taking these drugs after consulting their doctor, as it is dangerous to stop medication without medical advice.

FDA has found no evidence of acid reducers, such as, Zantac, Pepcid, or Axid interfering with Plavix.