Reputable online pharmacies should now be carrying this logo
More than two million people in the UK now buy their drugs online, research published by the Royal Pharmaceutical Society (RPSGB) suggests.
Up to a third of these medicines may be fake and could seriously damage people's health, it warns.
The society has launched a new logo for online chemists so shoppers know which businesses are reputable.
But many people want to purchase pills without prescription, and are likely to continue to visit unregulated sites.
According to the RPSGB, the most popular drugs sold online are Prozac for depression, Viagra for impotence and Valium to sleep.
It is possible to obtain a prescription online, as long as that prescription is signed by a doctor. But many circumvent the process and go straight to an internet dealer, even though it is illegal to provide prescription drugs without a prescription.
There is little the RPSGB can do to protect these people, but warn them of the dangers of taking drugs which could be at best useless and at worst positively harmful, the society said.
But what it can do is try to protect people with bona fide prescriptions or those who are buying over-the-counter drugs and who are shopping around for the best available price.
Local pharmacy 'safest'
The RPSGB logo will be clearly visible on the homepage of legitimate UK online pharmacies.
A link will take the user to the society's registration page, where they can check the chemists' credentials and those of the pharmacists who run the business.
"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," said David Pruce, director of Practice and Quality Improvement at the society.
The Medicines and Healthcare products Regulatory Agency (MHRA) said the safest way to obtain drugs was via prescription from your GP and bought at your local pharmacy.
"Internet websites are a common source of unlicensed and counterfeit medicine, and consumers should exercise caution when considering the purchase of medicines in this way," a spokesperson said.
But for those who wanted to shop online, "the RPSGB scheme provides reassurance that the pharmacy is registered and staffed by properly qualified pharmacists".
David Fisher, of the Association of the British Pharmaceutical Industry, said: "The pharmaceutical industry very much hopes that this initiative will go some way to answering some of the questions raised by the use of internet pharmacies.
"However, it is not going to solve all these problems overnight, and patients who order medicines through them must continue to take every precaution to ensure that their health is not put at risk."