The 18-year-old first-year student’s disappearance in January sparked a search that ended almost three months later when his body was spotted in a field, miles from his halls of residence.
Police say the teenager was drunk and suffered hypothermia as he tried to find his way home, but his mother insists her son’s drinks were spiked, causing his disorientated state.
Police Scotland said inquiries were continuing to establish the source of the GHB.
David O’Halloran’s mother, Donna, launched a campaign this month to create new licensing legislation she hopes will ensure the safety of young drinkers.
The proposals include enforced limits on the number of drinks per customer in bars and nightclubs, dedicated “cool-off” areas for anyone too drunk, and coasters and mats that can show if drinks have been spiked.
O’Halloran, 41, has launched an online petition demanding change and has already gathered more than 1,100 signatures.
David, who was studying maths and education, went missing in the early hours of 18 January, having decided to leave a taxi early on his way back from a night out with friends in Stirling.
He was spotted jogging towards the university, but then ventured through a field, arriving at a nearby dairy farm wet and with cuts on his hands from clambering over barbed wire.
His body was later found in a field by a farmer.
At the time Mr O’Halloran’s body was found, police said there were no suspicious circumstances surrounding his death.