One-in-eight people has taken a prescription medicine not prescribed for them, according to a study by the Health Products Regulatory Authority.
It says this represents about 440,000 people and is a major concern.
One-in-four people admit to not reading the information that comes with their medicine, the survey found.
This represents over 800,000 people and the authority says the number is increasing when compared to previous studies.
The authority has launched a campaign for the safe and effective use of medicines, both over the counter and prescribed drugs.
Chief Executive Lorraine Nolan said people need to inform themselves about the correct dose, directions for use and known side-effects of medicines.
She advised people to take just three minutes to inform themselves about their drug, to get the full benefit of medicines.
Ms Nolan said that significant details, such as the contraindications or potential side effects, can change from time to time, so it is important that those on long-term medication keep themselves informed.
At the launch, Minister for Health Simon Harris said he was very concerned about the figures, which show that more people do not read the information material with their medicines than a decade ago.