(Reuters) - The first U.S. treatment for low sexual desire in women, dubbed “female Viagra,” is more likely to help build a market for better future rival drugs than achieve the sales seen for Pfizer Inc’s famous little blue pill for men, industry experts said.

The U.S. Food and Drug Administration late on Tuesday approved the drug, called Addyi, for pre-menopausal women. The agency had previously rejected the treatment twice over concerns it did not provide a significant enough benefit to outweigh its risks, but came under pressure from advocacy groups who argued that women were unfairly denied access to the therapy.

When Addyi is introduced in mid-October by privately held Sprout Pharmaceuticals, it will carry strong warnings that it can cause severely low blood pressure and fainting, especially when used with alcohol, and pose serious risk to those with liver impairment or who take a class of drugs that include some steroids. It must be taken daily.

The dangers will likely deter many women since Addyi in clinical trials was associated with an increase of only one sexually satisfying sexual event per month, said Raghuram Selvaraju, managing director of brokerage H.C. Wainwright & Co.

"This drug is not going to provide a dramatic or meaningful improvement to a woman's sex life, so why would you take it with all these side effects?" said Selvaraju, who predicted Addyi will achieve peak annual U.S. sales of $100 million.

That's a far cry from the $1.3 billion in annual U.S. sales for Viagra, Pfizer's erectile dysfunction drug that was approved in 1998 and has blazed the path for lucrative rivals, including Eli Lilly and Co's Cialis.

There will be some pent-up demand for Addyi for perhaps six months, but then patients and doctors will abandon the drug as side effects pile up, Selvaraju predicted.

"When all is said and done, revenues from Addyi may wind up barely covering the costs of developing it," he said.

Len Yaffe, managing director of healthcare hedge fund Kessef Capital Management, predicts Addyi's side effects and slight efficacy will prevent its annual sales from topping $300 million.

"I don't expect it to become a Viagra for women," he said. "The most important thing is that it will open the door for approval of other drugs" to treat the condition, formally known as hypoactive sexual desire disorder (HSDD). Sprout estimates that up to 10 percent of U.S. women have the condition.

The price of Addyi has not been announced, but company officials on Wednesday said insurers are likely to require monthly co-payments of about $30 to $75, similar to those for Viagra.

An experimental treatment from Palatin Technologies, called bremelanotide, acts more quickly than Addyi. Like Viagra, it is taken only as needed and has been well tolerated, and it could capture up to $1 billion in annual sales if successful in ongoing late-stage trials, Yaffe said. That prospect could prompt a large drugmaker to buy Palatin or license its drug, he added.

Sprout executives said they plan to expand the market for Addyi by seeking approval outside the United States, but declined to speculate when. Having already studied Addyi in postmenopausal women, a potentially larger market, the company said it expects to seek approval for that indication in the future.

Dr. Rebecca Zucconi, a gynecologist and assistant professor at Quinnipiac University, said Addyi, while not a highly impressive drug, provides an alternative to the counseling and psychotherapy that have been a mainstay of treatment for women with low sexual desire.

"Hopefully we'll have several other options down the road" for women, she said. Addyi “is the first, and if it opens up the conversation, that's a good thing."

(Reporting by Ransdell Pierson; Editing by Bernard Orr)