Pfizer’s big-selling antibiotic Zithromax (azithromycin) is finally facing generic competition in the USA, after the Food and Drug Administration (FDA) gave the go-ahead to three manufacturers to start selling the drug.

The two largest generics houses - Israel’s Teva Pharmaceutical Industries and Novartis unit Sandoz – were among those given the green light, as was Croatian company Pliva which originally developed the antibiotic and is contracted to supply Pfizer with the active pharmaceutical ingredient (API) for Zithromax.

Pliva has chosen to develop and market its own version of the drug in order to offset the impact of its revenues caused by the loss of royalties on Zithromax sales. The company, which is in the throes of an exit from the proprietary medicines business, has also just won a three-year contract from Pfizer to continue to make the API.

The approvals are for 250mg, 500mg and 600mg tablet versions of the drug. Pfizer also sells Zithromax capsules, as well as oral and injectable versions of the drug.

Sales of the branded version are running at around $1.8 billion dollars a year, and are expected to dip sharply once the generics are launched.

Pfizer is also facing generic competition for blood pressure treatment Norvasc (amlodipine), the antifungal agent Diflucan (fluconazole), epilepsy drug Neurontin (gabapentin) and the antidepressant Zoloft (sertraline).