Trump hair
Dr Harold Bornstein told New York Times that Trump takes a small handful of pills daily, including finasteride to treat male pattern baldnessGetty Images

A drug taken by US President Donald Trump to treat hair loss has been linked to erectile dysfunction in a new study.

In February Trump's former physician, Dr Harold Bornstein, told the New York Times that Trump takes a small handful of pills daily, including the drug finasteride to treat male pattern baldness.

According to a recent study by Northwestern University, men with a longer exposure to the drug have a higher risk of getting persistent erectile dysfunction than those with less exposure.

For some subjects the persistent erectile dysfunction persisted despite stopping taking the drugs, in some cases for months or years.

Trump's health has previously been the subject of speculation.

When the 70-year-old announced he was running for office, Dr Bornstein released a letter claiming that "If elected, Mr Trump, I can state unequivocally, will be the healthiest individual ever elected to the presidency."

Dr Bornstein, who claims to have been Trump's personal physician for three decades, subsequently said that he had written the note in all of five minutes while a Trump limousine waited for him to finish.

The White House did not comment on Dr Borstein's claims.

Defying the so called Goldwater Rule, which prohibits psychiatrists evaluating a presidential candidate's mental health without a face-to-face consultation, mental health professionals signed a letter in the New York Times declaring "grave emotional instability indicated by Mr Trump's speech and actions makes him incapable of serving safely as president."

The new study is not the first time finasteride has been associated with negative side-effects, and at least 1,245 lawsuits have been filed against Propecia's manufacturer, Merck, alleging that the company didn't sufficiently warn users of sexual and cognitive negative consequences.

Among the subjects of the recent study, 167 of 11,909 (1.4 percent) developed persistent erectile dysfunction that continued for a median of 1,348 days after stopping finasteride and dutasteride, another male hormone blocker.