* Firms say drug will be in U.S. pharmacies by the year end
* Say FDA backs Potiga as a controlled substance
* Final classification still under review (Adds details, quotes)
LONDON, June 13 (Reuters) - The U.S. Food and Drug Administration gave its limited approval on Monday for GlaxoSmithKline's (GSK.L) and Valeant Pharmaceuticals International's (VRX.TO) new epilepsy drug Potiga.
The drug, known generically as ezogabine, will not be available immediately since the FDA has recommended it be listed under the Controlled Substances Act, the two drugmakers said.
"Final classification is still under review by the Federal Drug Enforcement Administration (DEA), and ezogabine will not be available until this process is complete," they said.
Ezogabine, which the FDA is approving as adjunctive treatment of partial-onset seizures in patients aged over 18, is expected to be available in pharmacies in the United States by the end of the year.
Potiga had appeared on its way to approval last August after an FDA expert advisory panel voted it was effective for use with other epilepsy drugs in patients who still have seizures with current therapy, and said monitoring for urinary problems would help minimise side effects. Potiga caused urinary retention in some patients during clinical trials.
But the FDA subsequently delayed its decision on the drug several times until issuing Monday's decision. The drugmakers said the FDA wants the drug to be used only in conjunction with a so-called Risk Evaluation and Mitigation Strategy (REMS), which will aim to inform doctors of the risk of urinary retention and the symptoms of acute urinary retention.
Analysts have forecast peak annual sales for Potiga ranging from $200 million to $800 million.
"We are so pleased to reach such an important milestone," said Susan Hall, head of research and development at Valeant.
"We believe this product will play a needed role in the management of partial onset seizures in appropriate patients who are uncontrolled on their current medications." (Reporting by Kate Kelland; Editing by Will Waterman)