The 45-year-old, named by Il Messaggero as Andrea, underwent penile injection therapy at a private clinic in Rome in February to treat impotence.

He was injected with "a small amount" of prostaglandin E1, a procedure that relaxes muscle tissue and enhances blood flow to the penis, the newspaper reported.

But the treatment, which started to work within a day, soon had the opposite effect to his previous ailment: his penis stayed erect for 36 hours at a time, for more than a month, causing him “excruciating pain”, the newspaper reported.

The period was “a nightmare” for Andrea and once the problem subsided, he decided to sue the two doctors, a urologist and an andrologist.

The doctors now face charges of “incompetence and negligence, for failing to adequately inform the patient that the treatment could induce a prolonged and painful erection,” Attilio Pisani, the prosecutor, was quoted in Il Messaggero as saying.

The potential for a prolonged erection is considered to be the most serious side-effect of prostagladin E1, according to the US National Library of Medicine. The problem, however, can be mitigated “by careful titration of the dose and through patient education”.

In 2012, Henry Wolf, an American, tried to sue BMW after he claimed a motorbike seat left him with an erection that lasted almost two years. But the case was dismissed after the judge in the US ruled that there was not enough supporting evidence, despite a urologist testifying that Wolf had priapism, the medical term for a penis that does not return to its flaccid state, claiming the condition was provoked by the motorbike’s vibration.