Last week, the FDA announced that it is investigating complaints about a generic version of GlaxoSmithKline's (NYSE:GSK) Wellbutrin XL. While that's not a good sign for Teva Pharmaceuticals (NASDAQ:TEVA), which markets the generic drug, I'm not sure it will have much effect on the slumping sales of Wellbutrin XL.

The XL (extended release) version of the antidepressant developed in collaboration with Biovail (NYSE:BVF) releases the drug slowly and only needs to be taken once a day. Users of Teva's Budeprion XL have been complaining about headaches, anxiety, and other ailments, which persuaded a consumer-product testing group to compare the two versions. The group found that Budeprion XL released 34% of its active ingredients after two hours, compared with just 8% for the original formula. That spike of drug entering the bloodstream could certainly explain the side effects.

The FDA action probably won't be much help for the sales of Wellbutrin XL, which slipped from $364 million in the fourth quarter of last year to just $231 million in the second quarter after generics were introduced in December. Watson Pharmaceuticals (NYSE:WPI) still sells its generic version of the drug, so even if Budeprion XL is pulled from the market, the brand name will still experience competition. The best Glaxo and Biovail can hope for is that the FDA takes its time reviewing the approvals for the 150 mg version of the drug, which, under an agreement with the generic-drug makers, can't be launched in generic form until next year.

Perhaps the biggest winners of the investigation are drugmakers such as Alkermes (NASDAQ:ALKS), DURECT (NASDAQ:DRRX), and Flamel Technologies (NASDAQ:FLML) that specialize in helping pharmaceutical companies develop extended-release drugs. If the FDA increases its overall scrutiny of generic versions of extended-release drugs, the companies might be able to continue reaping royalties beyond the patent expirations.

Extended Foolishness about extended-release drugs:

Flamel is a Motley Fool Hidden Gems newsletter pick. If small caps aren't your thing, why not pick up a 30-day free trial of our Income Investor newsletter, where you'll find Glaxo among the recommendations.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool has a disclosure policy.