The Food and Drug Administration announced last fall that two of the three available generic versions of the attention deficit hyperactivity drug Concerta are not as effective as the brand-name medication and could not be automatically substituted by pharmacies.

But seven months later the generics, one made by Mallinckrodt Pharmaceuticals and UCB, remain on the market and are still being sold, The New York Times reports.

The agency said that the drugs to be safe, but that their effectiveness wears off much more quickly — after about seven hours. The agency allowed the companies to continue selling their drugs, but gave them six months to prove the drugs were equivalent, or remove them from the market.

But major pharmacies like CVS and Rite Aid continue to stock them, and together the drugs have managed to hold on to about 30 percent of the market, The Times reports.