The first scientific evidence of a link between a drug for severe acne and depression was unveiled yesterday, following years of denials by the manufacturer that the prescription medicine could be in any way responsible for suicides.

Roaccutane has been linked to more than 100 suicides and attempted suicides worldwide. In April last year, an inquest heard that Jason Spiller, 16, killed himself after starting the drug. The coroner said, as have other coroners, that there was no clear evidence of a relationship between Roaccutane and depression. The previous year, David Roberts, a 20-year-old, killed himself while on the drug. Seumas Todd, 20, son of the actor Richard Todd, killed himself in 1997 while taking Roaccutane.

Roche, the manufacturer, maintains that the drug is not the problem, pointing out that the underlying condition could be to blame. The drug's information leaflet has, since 1998, warned patients may be at risk of becoming suicidal, at the request of the Medicines and Healthcare Products Regulatory Authority (MHRA).

But yesterday, scientists from Bath University, with the University of Texas, revealed that they have conducted experiments on mice which show that the drug makes them behave in a way that is recognised as the result of depression.

Sarah Bailey from the department of pharmacy and pharmacology at Bath, said that more research would be needed before anybody could be sure the same effect occurs in humans on the drug.

"Establishing a link between the active molecules within the drug and a change in depression-related behaviour, albeit in mice, is an important step forward in our understanding of the effects of this drug," she said. "To date the only evidence for any link with patients has come from individual case reports and such patient data is complicated by the psychosocial effects of having severe acne."

The researchers injected mice with Roaccutane for six weeks to achieve similar levels in the blood to those of humans taking the tablets. Then they put them through tests recognised as models for measuring the effects of depression in rodents. One was a "forced swim" test, where the animals were placed in water. The time they spent attempting to climb out was compared with the behaviour of mice that had not been given the drug.

The mice on Roaccutane spent longer immobile, without attempting to escape - a recognised sign of depressed behaviour - the researchers report in the journal Neuropsychopharmacology.

The father of Liam Grant, 19, from Dublin, is suing Roche in an attempt to prove Roaccutane caused his death. Mr Grant - also called Liam - has spent £340,000 on studies, including one published by the American Academy of Psychiatry, which scanned the brains of 15 people on Roaccutane and 15 on other acne medication. They found changes in part of the brain associated with depression. Mr Grant has been offered, without any admission of liability, a settlement of all his costs and the maximum compensation payable in the event of winning his case by Roche - around £800,000 - but he has refused to take it. Roche said it has not had time to examine the new results.

Backstory

Roaccutane is intended for patients over the age of 12 with severe acne and is sometimes prescribed when antibiotics have failed. It is known to damage the foetus, so women of an age to become pregnant can only have a month's supply at a time and may have to undergo a pregnancy test. Concerns over links to depression and suicide have been around for some years. The Medicines and Healthcare Products Regulatory Authority has received 1,588 reports of suspected adverse events experienced by people taking the drug between licensing in 1983 and September this year. In 38 cases, people died and 25 of those were suicide. The MHRA says the safety of roaccutane is under constant scrutiny. In June last year an expert working group recommended patients be monitored for signs of depression.