A new report says over half of people who died in drug related deaths in 2011 had taken more than one substance.
The figures are contained in a study published by the Health Research Board today.
The HRB National Drug-Related Deaths Index also revealed an increase in the number of drug related deaths between 2010 and 2011 - from 597 to 607.
Overdoses accounted for 365 of the cases - while drugs were the contributing factor in 242 incidents.
Senior Researcher at the HRB Dr Suzi Lyons said deaths from illegal drugs reduced during the period.
"There's been a sustained reduction in heroin-related deaths since 2009, from 115 deaths to only 60 in 2011," Dr Lyons said.
"There's also been a very significant decrease in the number of deaths where cocaine was implemented, from 66 deaths in 2007 to 23 deaths in 2011.
"And it's stabilised between 2010 and 2011."
The index provides the latest data about the nature and the extent of premature death due to problem drug and alcohol use in Ireland. It aims to help focus efforts to tackle problem substance use in an informed, evidence-based way in order to improve and save lives