The Royal Pharmaceutical Society of Great Britain (RPSGB) has created a visual logo to identify rogue internet sites selling counterfeit medicines.
The logo will be visible on the homepage of participating online legitimate pharmacies.
Research from the RPSGB shows that about two million Brits buy prescription-only medicines over the internet.
Almost a third of those surveyed admitted to not understanding online regulations.
The RPSGB says that while a number of legitimate sites provide online pharmacy services, there are also a number of suppliers operating from websites offering to sell medicines with no professional or healthcare expertise.
RPSGB Director of Practice and Quality Improvement said: "The internet presents a real danger to people's health.
"Dishonest traders are selling medicines online without any relevant professional qualifications or expertise. The products they sell can be poor quality at best and dangerous at worst.
"People who wish to buy medicines over the net should do it at a legitimate registered online pharmacy.
"The public must be vigilant and we recommend that they use the presence of this logo in conjunction with a routine of checks when deciding whether or not it is safe to buy."