The Swine flu vaccine has topped the list of drugs responsible for the largest number of suspected adverse reactions, new figures show.
A total of 779 suspected adverse reactions were linked to the vaccine between 2010 and 2011.
Most of these were relatively mild, such as localised swelling, gastrointestinal problems and flu-like symptoms.
However, health authorities have identified up to 30 cases of young people with the sleeping disorder narcolepsy as part of an investigation into a possible link between the condition and the swine flu vaccine, Pandemrix.
Health authorities insist the vaccine is safe and that the risk-benefit balance for the jab remains positive.
The Irish Medicines Board said the high reporting rate linked with the pandemic vaccine reflected "both the extent of usage and repeated requests and reminders by the IMB and Health Service Executive for reporting of experience with their use".
It added that the place of a medicine on an overall list of adverse reactions could not be taken as an indicator of safety or risk.
"The number of reports received cannot be used as a basis for determining the incidence of a reaction as neither the total number of reactions occurring, nor the number of patients using a medicine is known," it said in a statement.
Overall, figures requested by The Irish Times show there were just over 6,000 adverse reaction reports from January 2010 to December 2011.
The anti-psychotic drug Clozapine, also known as Clozaril, ranks second-highest in the number of adverse reactions (543).
Common adverse effects of the drug include dizziness, rapid heartbeat, constipation, excess saliva production and weight gain.
The vaccine that protects girls against cervical cancer - the HPV vaccine - was responsible for the third highest number of reactions (507), followed by the vaccines used in childhood immunisation (379).
The Health Service Executive, meanwhile, has said that it is working to identify any potential cases of narcolepsy as part of ongoing clinical research into any possible link.
Preliminary data from Finland and Sweden in 2010 linked an increase in narcolepsy among young people with Pandemrix.
Swedish data estimated the vaccination might cause three cases of narcolepsy for every 100,000 vaccinations. However, a more recent study has suggested that narcolepsy is triggered by the swine flu infection and not by the vaccine.
The young people in Ireland affected range in age from five to 21 years old. HSE clinics administered more than 900,000 doses of Pandemrix in 2009 and 2010.
The use of Pandemrix is no longer recommended in Ireland and GPs have been advised to return any remaining stocks. This year's seasonal flu vaccine does not contain Pandemrix.
Authorities in Finland have begun a compensation process that includes an initial payment and ongoing assessment of needs.