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Wyeth Pharmaceuticals Inc. may have thought that their biggest drug, Effexor XR, was safe from competition for a while but Sun Pharmaceuticals may be cutting into the revenue pie.

Wyeth holds the patent on Effexor XR capsules until July 2010 which means no other company can sell a generic version until that time. However, Sun may have found a way around it by producing its generic version of Effexor in tablet form. While both the capsule and tablet have the exact same ingredients, the simple change in pill format should allow Sun to get around patent protection when Wyeth's patent on the drug's main ingredient, venlafazine, expires in June 2008.

Wyeth has implied that it expects little impact from the generic pills as they are unlikely to get FDA certification as the exact equivalent of Effexor XR and thus won't benefit state laws that require generics to automatically be substituted for the more expensive branded drug. That would mean that doctors have to write out a new prescription for patients specifically for the generic Sun drug.

"It's more than a difference of being a capsule versus a tablet," Joseph Mahady, Wyeth senior vice president, told investors at a conference last month. "It is likely to be a very different technology. It may indeed be a very different delivery profile of the product."

"The challenge is clear" because of the alternative formulation, Uday Baldota, Sun's vice president of investor relations, said in an interview. "The effort on promotion and marketing will be different from what you will do on a fully substitutable generic product."

Effexor XR is indicated in the treatment of depression and anxiety.