Hair loss drugs can cause erectile dysfunction in men, scientists warn.

Those taking a popular growth stimulant were left impotent for an average of four years after finishing the medication, a study found.

Sufferers were left unable to maintain an erection despite being given Viagra to try and solve their problem.

Experts now say that taking finasteride is a bigger risk factor for the condition than diabetes, high blood pressure and smoking.

Those taking a popular hair growth stimulant were left impotent for an average of four years after achieving hair growth, a study found

The drug, sold as Propecia, lowers prostate specific antigen levels and is used for treating male-pattern hair loss.

Researchers from Northwestern University Feinberg School of Medicine assessed the effects of the hair growth stimulant, taken by Donald Trump, and dutasteride.

FINASTERIDE: WHAT IS THIS DRUG? Finasteride was originally developed to treat urinary problems in men. Studies showed the drug made prostate glands smaller by reducing the levels of the hormone dihydrotestosterone in participants. But during the clinical trials, scientists saw an unexpected side effect - hair growth. And so in 1997, the FDA approved the steroid inhibitor as the first ever drug to treat male pattern baldness. Taken once a day, the drug is mainly sold under the brand name Propecia. Millions of American adults use the pills, which are proven to be 90 per cent effective. It is unsure how many men in the UK use it as it is a prescription-only treatment that isn't available on the NHS. Other than impotence, some side effects include a rash or tenderness around the nipples. Advertisement

Study author Dr Steven Belknap said: 'Men who take finasteride or dutasteride can get persistent erectile dysfunction, in which they will not be able to have normal erections for months or years.'

Both are male hormone blockers that prevent testosterone from being converted to its more active form.

The former is prescribed to some men with baldness and also sold under the brand name Proscar.

While the latter, more commonly known as Avodart, is used primarily to shrink the size of prostates.

Of the 11,909 men who were studied, 1.4 per cent went on to develop persistent erectile dysfunction. This continued for an average of 1,348 days.

Prior to this study published in PeerJ, research was scarce on the sexual effects of taking finasteride and dutasteride.

There was also no strong evidence that taking either for a long period of time increased the risk of impotence.

But the researchers found men under the age of 42 who used either drug for seven months had a 4.9-fold higher risk.

This comes after Turkish scientists last week found blood type could influence a man's performance under the sheets.

Those with blood types A, B or AB are up to four times more likely to suffer from impotence than men who have blood type O.

The findings were significant as it’s estimated more than half of all men carry the three danger types.