An inquest into the death heard Mr Long, an engineer from Atherton, Greater Manchester, had been prescribed Roaccutane for severe acne.
But he suffered had a severe reaction to the drug and subsequently began to suffer serious mental health problems.
He was referred to consultant psychiatrist Dr Ali Malik who said Mr Long had become disillusioned and depressed after taking Roaccutane.
Dr Malik, who treated Mr Long for nine months before his death, told thge inquest in Bolton:: "Although he finished this treatment some time ago, I believe it may be relevant to the inquest."
Mr Long was diagnosed as suffering from schizophrenia and was prescribed an anti-psychotic drug to manage the illness.
"In March, 2004, it was noted that Mr Long drove around for four hours to avoid his birthday celebrations.
"He refused to drink water from the tap in case it was poisoned and he believed there were hidden cameras in the house and one eyeball was not real and didn't belong to him."
On January 18, 2006, Mr Long parked his white Fiat Punto car, lay on the tracks near a bridge in Atherton and was hit by a train.
The eight-member inquest jury heard train driver, Tony Sulcas, applied the breaks and sounded the horn but Mr Long did not move from the tracks.
He said: "He would have had plenty of time to get up from the tracks before the train got to him but he didn't move.
"The train stopped a bit further up the track and I went back to Adam to see if there was anything I could do but there wasn't."
The family broke down in tears when Sergeant Leslie Peters of British Transport Police told the inquest Mr Long's mobile phone was found on the tracks with a text message which read: 'I love my mum and dad' on the screen.
Recording a verdict of suicide, Coroner Jennifer Leeming said she would be filing a report the adverse drug reaction to medical authorities.
She said: "I do not believe as a society that we take mental illness seriously. We do not understand that you can die from mental illness just as you can die from cancer. It is a lesson to us all."
A spokesman for Roche said: "Unfortunately, severe acne can cause some sufferers to become depressed and can also affect their mood and self esteem.
"This is why the information provided with Roaccutane carries a warning that some patient may experience mood changes, including an increase in depression.
"While none of this alters the terrible impact that Adam's suicide has had, it is worth remembering that over the past 20 years, more than 13 million people worldwide have been successfully treated for severe acne using Roaccutane."
In Britain up to 2,500 people use Roaccutane, which tests show helps alleviate acne in 89 per cent of sufferers.
Last night Mr Long's parents said their son had been on a five month course of the acne drug when he was 18.
His mother Sonia said: "He died 14 months ago and this inquest had just dragged it all back up again. It's just too upsetting. We're tying to get over it."
His father Stephen said Adam was their only son and had three older sisters.
He said: "We've not got a medical background, we don't know what's to blame, we just need time to get over this."