Almost a million units of illegal medicines were detained here in 2017, according to the Health Products Regulatory Authority (HPRA).
The figure is more than a 40% increase on the previous year.
Almost half of the products detained were anabolic steroids, followed by sedatives and erectile dysfunction medicines.
The HPRA says it is concerned at the increase and is warning people of the health risks of taking unauthorised medicines.
John Lynch, Director of Compliance at the HPRA, said: “It is extremely concerning to see this year-on-year increase in detentions which indicates that people are continuing to put their health in the hands of individuals who operate unauthorised supply routes.
"We also know that there is absolutely no guarantee that these products contain the type or quantity of active ingredient that’s stated on the label.
“We are particularly concerned at the rate of increase in detentions of anabolic steroids which indicates a demand among some people for these highly potent prescription medicines."
He outlined the effects of long-term use of anabolic steroids.
Mr Lynch said: "They have been linked to a range of significant side-effects including liver damage, blood clots, heart attacks and strokes.
"For men, the risks also include reduced sperm count and infertility.
"We would urge anyone using illegally sourced anabolic steroids to stop doing so and to seek advice from their healthcare professional if they have any concerns about their health.”