The Health Products Regulatory Authority (HPRA) has published details of illegal product detentions for 2016.
A total of 673,906 dosage units of falsified and other illegal medicines were detained by the HPRA during the year, compared to 1.1 million units in 1015.
The illegal products included sedatives (40%), anabolic steroids (16%) and erectile dysfunction medicines (14%).
The HPRA confirms that those behind the websites supplying these products are engaged in illegal activity and it stressed that, from a consumer health perspective, there can be no guarantees about the safety or quality of prescription medicines sourced online.
Commenting on the enforcement figures for 2016, John Lynch, Director of Compliance, HPRA said: “It remains a serious concern that people are sourcing medicines from unknown and potentially untrustworthy sources.
"There is absolutely no guarantee that they contain the type or quantity of active substance they say that they do. In some cases they can contain different substances altogether. They can be very dangerous to human health. We emphasise that, while some sites may appear legitimate, they are, too often, a front for illegal activity.
“It is particularly noteworthy that anabolic steroids accounted for 16% of products detained, a significant increase compared to 2015. The misuse of anabolic steroids has been linked to a range of significant side effects including blood clots, headaches, depression, irritability and stomach pain.
"People are sourcing these products for purposes of performance enhancement and are either unaware of or ignoring the significant dangers posed by these prescription medicines in the absence of medical supervision," he said.