Legal medications, such as paracetamol, psychotropic drugs, tranquillisers and sedatives, along with alcohol and illegal drugs, accounted for the bulk of substances used in overdoses.
Unpublished data from the Hospital In-Patient Enquiry (HIPE) scheme shows there were 4,254 overdose cases in 2011, with women accounting for 54% of incidents.
The number is down on 2010 (4,522) and significantly down on 2005 (5,012).
All the age groups experienced falls in cases in 2011, except the 45-54 and the 55-64 age groups.
The figures, contained in a report by the Health Research Board, show that 467 children aged under 14 experienced overdoses in 2011, along with 936 young people, aged between 15-24, the category with the highest number of cases.
Non-opioid analgesics, including pain killers, were present in 1,336 cases (31%), with paracetamol making up 806 of those.
Prescribed tranquillisers and anti-anxiety medication, known as benzodiazepines, accounted for 960 cases, with psychotropic agents making up 973 cases.
Alcohol was involved in 518 cases, but alcohol was only taken into account when it was used in conjunction with other substances.
Narcotic or hallucinogenic drugs were involved in 580 (14%) cases. Opiates, such as heroin and methadone, accounting for 445, cocaine for 82 and cannabis for 46.
Of 4,174 cases, some 2,773 (66%) were classified as intentional, 885 accidental and 516 undetermined.
Again, the main substances involved were non-opioid analgesics, benzodiazepines and psychotropic agents.
The HRB report also shows there were 839 admissions to Irish psychiatric units and hospitals in 2011 with people suffering a drug disorder, of whom 352 were treated for the first time.
The report cites findings of research conducted among young people under the age of 18 attending the country’s largest outpatient adolescent drug treatment programme in Dublin.
Some 144 admission assessments to YoDA (Youth Drug and Alcohol Service) were examined during an 18-month period in 2010 and 2011.
The main drug of addiction was cannabis (89) followed by alcohol (33).
The research found that 55 (38%) had pre-existing psychiatric disorders and 18 (13%) received a psychiatric diagnosis following entry to YoDA.