WASHINGTON (Reuters) - The pharmaceutical industry may stop investing in medicines to treat diseases like diabetes or obesity without more explicit guidelines from U.S. regulators, the chairman of the drug trade group said on Thursday.

The Food and Drug Administration must approve any medical products sold in the United States, but drug companies say they cannot always predict how the agency weighs risks and benefits for medicines that could be widely used.

To avoid the uncertainty, companies may focus on specialized cancer drugs, where it is clear patients and the FDA are willing to accept serious side-effects in exchange for potentially life-saving treatments.

"You're starting to see primary care diseases becoming somewhat neglected," said Christopher Viehbacher, chairman of the board of the Pharmaceutical Research and Manufacturers of America (PhRMA), in an interview.

"To make sure we're not ignoring unmet needs in primary care, we need a lot more clarity around the risk-benefit so there's predictability when we invest in these products," said Viehbacher, also chief executive of French drugmaker Sanofi SA.

PhRMA and other groups are gearing up their lobbying strategy ahead of Congressional hearings on FDA user fees, or the funds companies pay to the agency in exchange for faster review times.

Since fees from makers of drugs and medical devices provide more than a third of the agency's funding, the bill often serves as a vehicle for broader FDA-related changes.

Congress must renew the Prescription Drug User Fee Act (PDUFA) every five years, with the current legislation expiring in September 2012.

Over the past year, the FDA has drawn fire from some manufacturers for being too cautious in reviewing medical products, hindering U.S. innovation.

They point to obesity drugs, where Arena Pharmaceuticals, Orexigen Therapeutics and Vivus have hit roadblocks in gaining approval for their diet pills because of potential safety concerns.

About a third of U.S. adults are obese and medicines could help manage the weight along with diet and exercise.

Viehbacher said the FDA is aware of the challenge of balancing patient safety and innovation.

"You're hearing the FDA say that yes, we need to protect patients, and yes, we need to keep medicines accessible," he said. "I don't think anybody has to sacrifice. I think there just has to be a lot more science."

(Reporting by Anna Yukhananov; Editing by Matt Driskill)