By Toni Clarke

WASHINGTON (Reuters) - A drug to treat low female sexual desire should be approved with strict measures in place to ensure patients are fully aware of its risks, an advisory panel to the U.S. Food and Drug Administration concluded on Thursday.

Eighteen panelists voted in favor of approving the drug with a risk management program. Six voted against approval. None voted to approve the product without such a program.

The benefits of the drug are marginal, panelists said, but meaningful for some patients. Serious side effects include the risk of fainting at unpredictable times, accidental injury and low blood pressure.

The FDA, which has twice rejected the drug, flibanserin, is not obliged to follow the advice of its advisory panels but typically does so.

The panel's recommendation follows months of lobbying by the drug's developer, privately held Sprout Pharmaceuticals, aided by a number of women's advocacy groups which accused the FDA of gender bias, a charge the agency rejects.

The drug works differently from Viagra, which is used to treat erectile dysfunction and has been available since 1998. Flibanserin is a 5-HT1A receptor agonist and 5-HT2A receptor antagonist that acts on the brain, while Viagra affects blood flow to the genitals.

Potential risk management measures suggested by the panel included requiring physicians to be certified before being allowed to prescribe the drug and requiring pharmacies to confirm the physician's certification.

Recommendations also included establishment of a patient registry and additional safety studies after the drug is on the market. Some panelists recommended a warning against the use of alcohol when taking the drug.

Dozens of women spoke to the panel about the distress caused by their low sexual desire and urged the FDA to approve the drug, whose proposed trade name is Addyi.

Others characterized Sprout's lobbying campaign as an attempt to bully the FDA into approving a drug with modest benefits and real safety concerns. Some panelists said they were concerned that patients could faint while behind the wheel of car or in other circumstances that could lead to serious injury or death.

Flibanserin is a pill that must be taken daily. It was originally developed as an antidepressant by Boehringer Ingelheim, which sold the drug to Sprout following a negative advisory panel meeting in 2010.