Glaxo SmithKline, the maker of Alli, says it's found counterfeit versions of the 60 mg capsule/120 count refill kit.
Tests done by the company show the capsules didn't contain orlistat, the active ingredient in Alli.
Instead, they contained sibutramine, which is sold as a weight loss drug under the name Meridia.
Sibutramine is a controlled substance which shouldn't be used by some people, and should not be used without physician supervision.
Consumers began reporting suspected counterfeit Alli to Galxo SmithKline last month.
The company says it appears the bogus drug has been sold over the internet, but not in retail stores.
The counterfeit Alli can be recognized by these differences in packaging.
* Outer cardboard packaging missing a "Lot" code
* Expiration date that includes the month, day, and year (e.g., 06162010); authentic Alli expiration date includes only the month and year (e.g.,: 05/12)
*Packaging in a plastic bottle that has a slightly taller and wider cap with coarser ribbing than the genuine product
*Plain foil inner safety seal under the plastic cap without any printed words; the authentic product seal is printed with "SEALED for YOUR PROTECTION"
*Contains larger capsules with a white powder, instead of small white pellets.
If you may have received counterfeit Alli, please call the FDA's Office of Criminal Investigations by calling 800-551-3989 or by visiting the OCI Web site (http://www.fda.gov/OCI). Anyone is sickened or has a reaction to the medications is asked to call the FDA's MedWatch Program by phone at 1-800-FDA-1088, by fax at 1-800-FDA-0178, or by mail at: MedWatch, HF-2, FDA, 5600 Fishers Lane, Rockville, MD 20852-9787. Pictures of counterfeit Alli samples provided by GSK can be viewed at: FDA.gov