Complementary remedies claiming to treat erectile dysfunction (ED) can be dangerous, misleading and contain unspecified chemical ingredients, the United Kingdom drugs regulator has warned.

The Medicines and Healthcare products Regulatory Agency (MHRA) said patients were taking "major risks" by taking herbal remedies for ED and has urged patients to avoid buying them.

"Herbal remedies promoted for the treatment of erectile dysfunction are often sold via mail order, the internet or through traditional herbal medicine outlets," MHRA said in a statement.

"Many products falsely claim to be 100 per cent natural." MHRA urged patients to stick to Viagra (sildenafil), Cialis (tadalafil) and Levitra (vardenafil), which are prescribed by doctors and dispensed from pharmacies.

"Evidence from around the world suggests that such products are often adulterated with random quantities of pharmaceutical substances," MHRA said.

"Consumption of medicines containing random, uncontrolled quantities of these analogue chemical compounds is especially dangerous and could potentially cause serious adverse reactions such as heart attack, stroke and severe hypotension."

The agency said it tested 138 unlicensed ED treatments since 2005, finding prescription ingredients in widely ranging quantities in almost two-thirds of them.