LOS ANGELES - MANY of the pills marketed as safe herbal alternatives to Viagra and other prescription sex medications pose a hidden danger: For men on common heart and blood-pressure drugs, popping one could lead to a stroke, or even death.

'All-natural' products with names like Stamina-RX and Vigor-25 promise an apothecary's delight of rare Asian ingredients, but many work because they contain unregulated versions of the very pharmaceuticals they are supposed to replace.

That dirty secret represents a special danger for the millions of men who take nitrates - drugs prescribed to lower blood pressure and regulate heart disease.

When mixed, nitrates and impotency pharmaceuticals can slow blood flow catastrophically, leading to a heart attack or stroke.

An investigation shows that spiked herbal impotency pills are emerging as a major public health concern that officials haven't figured out how to track, much less tame.

Emergency rooms and poison control hot lines are starting to log more incidents of the long-ignored phenomenon.

Sales of 'natural sexual enhancers' are booming - rising to nearly US$400 million (S$ 582 million) last year. And dangerous knockoffs abound.

At greatest risk are the estimated 5.5 million American men who take nitrates - generally older and more likely to need help with erectile dysfunction.

The all-natural message can be appealing to such men, warned by their doctors and ubiquitous TV commercials not to take Viagra, Cialis or Levitra.

James Neal-Kababick, director of Oregon-based Flora Research Laboratories, said about 90 per cent of the hundreds of samples he has analyzed contained forms of patented pharmaceuticals - some with doses more than twice that of prescription erectile dysfunction medicine.

Other testers report similar results, particularly among pills that promise immediate results.

An elderly man in a retirement community north of Los Angeles took an in-the-mail sample and landed in the hospital for four days.

Tim Fulmer, a lawyer representing Spontane-ES, said the pill did not contain any pharmaceutical and was not responsible for the stroke.

Mark B. Mycyk, a Chicago emergency room doctor who directs Northwestern University's clinical toxicology research program, said he is seeing increasing numbers of patients who unwittingly took prescription-strength doses of the alternatives, a trend he attributes to ease of purchase on the Internet and the desperation of vulnerable men.

He said he wouldn't be surprised if there'd been undetected deaths from bad herbal pills.

Some herbal labels warn off users with heart or blood-pressure problems if they have taken their medicine within six hours; some doctors say 24 hrs or more would be safer.

The AP often couldn't determine from records whether incidents reported to tracking systems of the federal Food and Drug Administration and state poison control centers involved mixing herbal alternatives with nitrates.

Some men in their 30s who went to emergency rooms after taking herbal sex pills were presumably otherwise healthy, but they showed the transitory side effects of the active ingredients in regulated impotency pharmaceuticals, such as difficulty seeing clearly or severe headaches, records show.

While public health officials don't know the extent of the problem, they agree that incidents are vastly underreported, with national tracking systems capturing perhaps as little as 1 percent of them. Victims may be embarrassed, and doctors rarely ask about supplements.

Since 2001, sales of supplements marketed as natural sexual enhancers have risen US$100 million to US$398 million last year, including herbal mixtures, according to estimates by Nutrition Business Journal.

Some legitimate herbal mixtures claim to work gradually over weeks; it's the herbals marketed for immediate trysts that often are the problem.

Tight budgets, weak regulations and other priorities limit the FDA's ability to police the products, often promoted via blasts of e-mail spam and fly-by-night websites.

Linda Silvers, who leads an FDA team that targets fraudulent health products sold online said, ' a website can look sophisticated and legitimate, but actually be an illegal operation.'

In many cases, the ingredients used to alter herbal pills come from Asia, particularly China, where the sexual enhancers are cooked up in labs at the beginning of a winding supply chain.

The FDA has placed pills by two manufacturers in China and one from Malaysia on an import watch list.

Spiked pills have turned up in Thailand, Taiwan, Canada, Australia, New Zealand, Hong Kong, Malaysia, the United Kingdom and the United States, according to testing done by Pfizer Inc., the New York-based pharmaceutical giant that developed Viagra.

The company said that 69 per cent of 3,400 supplements it purchased in China contained sildenafil citrate, the main ingredient in Viagra.

Pfizer didn't check for the patented ingredients of its rivals.

While herbal alternatives often contain exact copies of the patented drugs, some makers tweak the molecules to keep the effect of the original pharmaceutical while avoiding the scrutiny of the FDA and outside testing labs.

During the past year, the FDA has orchestrated eight recalls of 'herbal' pills that contained the ingredients found in Viagra, Cialis or Levitra, or their unregulated chemical cousins. -- AP