Daniel Charles Riley, 39, is standing trial on two counts of manslaughter and three of poisoning after five of his acquaintances, drug users he met through boarding houses and mental institutions, allegedly became ill from taking the drugs Aurorix and Effexor.

The Crown claimed Riley knew the drugs had the potential to endanger lives because he had earlier been treated in hospital after taking them together and had witnessed another woman become seriously ill after taking the drugs four years before the first fatality.

He allegedly used different doctors and pharmacists to obtain the drugs because he knew no medical practitioner would allow him to take them together.

“The combination leads to an excess of serotonin in the brain, and such serotonin can be devastating,” the prosecutor, Ken McKay, told the jury.

Shaun Bateson, 22, died from serotonin toxicity after taking the drugs while living in a boarding house in Kirrawee with Riley in May 2003, the court heard.

Two weeks later, Riley moved to a lodge in Surry Hills and met Brian Hadfield, 29, who also died after taking the drugs in July 2004. An autopsy later found serotonin poisoning was the most likely cause of death.

Six months later he reunited with three friends he had met in a psychiatric hospital and allegedly went on a drug and alcohol-fuelled bender that ended with two of them in hospital.

Mr McKay said Riley knew the dangers of taking the drugs together but continued to do so, and encouraged others to do likewise.

But Riley’s barrister, Tania Evers, said all the alleged victims were drug addicts and there was no evidence he had encouraged them to take his antidepressants.

“These were people who were experimenting with drugs, who were addicted to drugs. They took drugs both prescribed and illegal; they even bragged about drugs,” Ms Evers said.

The trial started Monday and is expected to run for between five and six weeks.