Increasing numbers of people are buying potentially lethal prescription medicines over the internet without a prescription, pharmacists have said.
Three-quarters of pharmacists believe the activity has become more commonplace in recent years, according to a poll by the Royal Pharmaceutical Society (RPS) and pharmaceutical company Pfizer.
Officials warned that unlicensed or fake medicines could contain harmful ingredients such as arsenic and could potentially be lethal.
Neal Patel, of the RPS, said: "These are worrying statistics and it's clear from our members that patients are still unaware of the potential risks associated with purchasing medicines online from unregulated or unverified websites.
"Some of these illegal sites are very professional and look like legitimate online pharmacies, but supply dangerous fakes or unlicensed medicines that have serious health implications.
"Our advice is clear; always buy medicines in person or online from a genuine UK bricks and mortar based pharmacy."
Many thought people were opting to buy the medicines online to alleviate embarrassment, to access treatments quickly, or to self medicate.
The news comes as the Medicines and Healthcare products Regulatory Agency (MHRA) announced that in the past five years, officials have seized more than £34 million worth of medicine supplied illegally.
A group of health organisations have launched the Real Danger campaign to raise awareness about the risks of buying prescription medicines online through unregulated channels without a prescription.
Nimo Ahmed, head of enforcement at the MHRA, added: "Counterfeit and unlicensed medicines are potentially lethal - you have no assurances about ingredients, quality or how the medicines have been made.
"Despite these risks, our recent seizures of vast quantities of illegal medicines demonstrate that there is still a huge demand in the UK.
"This campaign aims to further educate the public so they are aware of the potential dangers and we can work towards halting this dangerous criminal market."
Dr Berkeley Phillips, medical director at Pfizer, said: "The harsh reality is that unlicensed or fake medicines, easily accessible online, can contain harmful ingredients such as arsenic.
"They are often produced by people with no appropriate qualifications and can contain no pharmaceutical ingredients at all.
Some fake medicines can contain totally different ingredients to the labelled active ingredients, some of which may interact with other medications, exacerbate other ailments or simply be toxic.
"Buying prescription-only medicines without a prescription has frightening consequences of which we want to make the public aware.
"Fake medicines can cause harm to patients, which can sometimes lead to death."