Fake prescription drugs for sexual enhancement are circulating in Australia and they can be "lethal".

Doctors have reported the recent case of a 54-year-old truck driver who was sweating profusely, dazed and slurring his speech when he was admitted to a Queensland hospital.

It was later confirmed he had taken a "Cialis 50" tablet, not the erectile dysfunction drug Cialis which is a rival to Viagra but a fake version that contained a dangerous cocktail of chemicals.

"This is the first report of a laboratory-confirmed counterfeit Cialis tablet in Australia," writes Dr Yong Tan, and colleagues from Townsville Hospital, in a paper published in the Medical Journal of Australia.

"... Medicines for erectile dysfunction or sexual enhancement have a huge global market, and this is not the first report of an adverse reaction."

The man was diagnosed with severe hypoglycaemia - low blood sugar - but its trigger remained unknown until the man attended a follow-up examination two weeks later.

He admitted that a friend had purchased a "bottle of Cialis 50" in Vietnam, and he had taken one of the tablets before the attack.

Doctors tracked down the suspect drug to confirm they were fake.

Their analysis also showed the tablets contained a "lethal dose of sulfonylurea" - which lowers blood sugar levels - among a range of chemicals not known to provide any boost to sexual performance.

"This was the first time he had taken any medication of this type," Dr Tan said.

"Our case reveals the poor quality-control measures used during the manufacturing process of counterfeit Cialis.

"(And that) consumption of counterfeit medicines may be harmful."

Dr Tan said it also highlighted the need for the Therapeutic Goods Administration to publish warnings on its website about the risks posed by fake sexual enhancement drugs.

Weight loss medications and steroids were also targeted by drug counterfeiters, he said, while overseas travel and imports organised via the internet were the key ways they entered the country.

The World Health Organisation (WHO) estimates that up to one per cent of all drugs in industrialised countries, and 10 per cent globally, may be counterfeit.