Counterfeit medicines pose an ever-increasing threat to public health, although precise tracking of illegal counterfeit prescription drug activity is difficult. Available data indicate that all types of medications have been targeted. Adverse health effects, including death, have resulted from using counterfeit medications; consumers who self-medicate without appropriate interactions with the healthcare system rarely receive adequate healthcare. The Internet provides a large, convenient route for counterfeiters to reach potential buyers with unregulated, often dangerous, products. The majority of medicines purchased via unverified Internet sites are counterfeit; often, these products lack the purported drug compound or have variable concentrations of active ingredients and sometimes contain dangerous toxins. Although many consumers acknowledge some degree of risk with purchasing medications via the Internet, speed, convenience and cost often prompt these purchases. Counterfeit medications also have been detected in the legitimate supply chain, but represent a significantly smaller proportion of sales than those purchased via the Internet. Pilot programmes in Europe have demonstrated that product verification systems prevent penetration of counterfeit products into the legitimate supply chain. Significant EU legislation, including stronger penalties for counterfeiting, is in development. In the United Kingdom, the Medicines and Healthcare Products Regulatory Agency (MHRA) launched an initiative against counterfeit medication. Healthcare professionals should report suspected cases of counterfeit medication to the MHRA, be alert to threats to the medicine supply, and provide practical advice to patients about ordering medications online, including avoiding unregulated Internet pharmacies, and being suspicious of sites offering substantial discounts and prescription-only medication without a prescription. © 2011 Blackwell Publishing Ltd.