A dispatch rider who died five days after he crashed into shrubs on an expressway had ridden under the influence of drugs.

This could have compromised his ability to control his motorcycle, said a coroner on Wednesday

Mr Sobirin Ahmad, 61, had lost control of his motorcycle on the Ayer Rajah Expressway on April 27. He veered to his left and crashed into the shrubs before landing on the road.

He was taken to the National University Hospital, where he was subsequently found to have a brain injury caused by oxygen deprivation. He was pronounced dead on May 2.

At an inquest into his death, the court heard that there were no tyre or skid marks.

Mr Sobirin had three traffic and parking-related violations between 2004 and 2009, with a speeding offence recorded in February this year. He also had criminal records for drug consumption.

In his findings, Coroner Marvin Bay said the manner in which Mr Sobirin lost control of his motorcycle could not be ascertained as there were no witnesses, but he added that "the presence of chloroquine, together with morphine and nitrazepam, in Mr Sobirin's post-mortem blood sample suggests illicit drug use on his part".

Nitrazepam and morphine can impair the ability to control a moving vehicle. Morphine can also cause drowsiness.

Coroner Bay said detectable traces could still be found in Mr Sobirin's autopsy sample taken five days after the incident. This may point to a "fairly high consumption" of the drugs, he said.

"Mr Sobirin's choice to ride, while in a compromised state of drug-induced stupor, was most unwise and for which he has sadly paid the ultimate price," said the coroner.

He found his death to be an unfortunate traffic misadventure.