RESEARCHERS have warned about "herbal remedies" for erectile dysfunction sold over-the-counter in Hong Kong after most of them were found to contain untested variants of well-known anti-impotence drugs.

These variants, or "analogues," are copies of controlled drugs but they are slightly modified in their chemical structure to escape patent and other drug-related laws.

Cranked out illegally, these chemicals are not tested for their efficacy or safety and can have unpredictable, adverse effects. They are added undeclared into "health products," which do not come under stringent tests in most countries.

Writing in the latest issue of the Hong Kong Medical Journal, the scientists said they studied 26 anti-impotence products sold in convenience stores and pharmacies.

Although their packaging declared that they contained only "herbal ingredients," variants of sildenafil and vardenafil were found in 14 of them. Sildenafil was found in one product.

Sildenafil and vardenafil are generic names for their better known trade labels Viagra and Levitra, respectively, and they are prescribed drugs in Hong Kong.

"The positive rate of concealed drug analogues in male erectile dysfunction health products is alarmingly high. Such analogues are difficult to detect by ordinary laboratory methods and might be used in an attempt to evade regulatory inspection," wrote the researchers at the Hospital Authority Toxicology Reference Laboratory in the journal's October issue.

"Without going through the stringent drug testing process, the adverse effects of these chemicals remain largely unknown and unpredictable," they warned, as they called for urgent and more effective surveillance and control.

This malpractice came to light when a previously healthy 28-year-old man got admitted to hospital for an unsteady gait and frequent falls - signs of ataxia.

Doctors then learnt that he had taken an anti-impotence "health product" for over a week before showing those symptoms.

Classified as phosphodiesterase-5 inhibitors, sildenafil and vardenafil are known to have side effects like nausea, headache, facial flushing and visual disturbances. Serious cardiovascular effects have also been reported.

But ataxia has never been linked to this class of drugs.