At least 11 men have died and more than 300 suffered nasty reactions after taking illegal sex drugs over the past six years.
The Health Sciences Authority (HSA) said the "adverse reactions" included dangerously low blood sugar levels and lapsing into a coma.
The oldest to land in hospital was a 97-year-old who obtained four pink tablets from a friend "for enjoyment with women". He took a pill at lunch and was found unconscious by his son.
Associate Professor Chan Cheng Leng, the HSA's deputy group director of Health Products Regulation, said: "He was lucky to survive as he had life-threatening low levels of blood sugar."
Men who buy sex drugs from street peddlers or online are putting themselves in harm's way, she added. "You never really know what the pill contains. The illegal drug manufacturers are unscrupulous."
Most of the deaths took place in 2008 and the last death was in 2009. But men are still landing in hospital with adverse reactions to these illicit drugs.
The drugs, often made in poor conditions without quality control, may contain too little of the active ingredient, rendering them ineffective. Too much, and they could be harmful. Some also contain dangerous or banned substances.
Take for example Ji Yin Yu Gen and Max Man Ultimate. Both drugs were found to have Sibutramine, which is banned as it can increase the risk of heart attack and stroke.
Dr Daphne Gardner, a consultant at the Singapore General Hospital's endocrinology department, co-authored a study of 150 patients admitted to hospital in 2008. All but one were men.
They suffered from dangerously low blood sugar levels, which rarely occurs in patients without diabetes.
A large number admitted that they had used illegal sex drugs, such as Power 1 Walnut, Santi Bovine Penis Erecting Capsule and Zhong Hua Niu Bian, to treat erectile dysfunction or impotence.
The drugs contained Glibenclamide, which is used to treat diabetes, and some contained dangerous amounts of the ingredient.
Seven men fell into a coma and four subsequently died. Some were left paralysed and needed long- term care, Dr Gardner said.
"Attempts to trace these drugs to the original manufacturing source did not yield fruit, since the drug packaging contained names of fictitious factories overseas," she added.
Some men continued to pop dubious sex pills even after they fell ill.
One 49-year-old was hospitalised three times. He was found unconscious the first two times, but that did not stop him from taking the pills again.
Although erectile dysfunction is usually associated with older men, doctors say younger men are also taking sex drugs to boost their performance.
A quarter of those who had bad reactions were below 39 years old. The youngest was an 18-year- old whose blood sugar levels plunged and he suffered seizures. He was hospitalised for three days.
Dr Tan Kok Kuan, a doctor in private practice, said he has patients as young as 18 who buy illegal sex drugs before visiting prostitutes in Geylang.
He said a Viagra pill costs about $20 at a clinic. Knock-off versions are available in Geylang for $2.50 or even cheaper.
But it is not just price that draws the men to street peddlers. Dr Tan said: "You don't have the embarrassment of telling your doctor that you have erectile dysfunction. I don't think these men think of the dangers."
This story was first published in The Straits Times on Sunday, Nov 23, 2014