According to research commissioned by pharmaceutical giant Pfizer out Tuesday, the market in counterfeit medicines in Europe is worth an estimated 10.5 billion euros a year.
One in five of those questioned for the research admitted buying prescription-only medicines without a prescription, with people in Germany (38 percent) and Italy (37 percent) the worst offenders.
Italians and Germans are also thought to have spent the most on fake prescription-only medicines -- an estimated 3.6 billion euros and 2.7 billion euros respectively.
The most popular kinds of medicines to buy are those related to weight loss (45 percent), flu (35 percent) and erectile dysfunction (25 percent), the research found.
It comes amid growing concern over the issue among officials who say fake medicines are often unsafe, inefficient or of poor quality.
In December European industry commissioner Gunter Verheugen said the European Union had seized 34 million fake tablets in just two months, a figure which "exceeded our worst fears".
An EU report from July last year claimed that many of the fake pharmaceuticals seized in 2008 came from India.
But the World Health Organisation says it is hard to calculate the extent of counterfeiting of medicines.
Research consultancy Nunwood questioned 14,000 people in 14 European countries online in November for the Pfizer research.
The 10.5 billion euro (14.2 billion dollar) figure was calculated based on the number of people buying prescription-only medication without a prescription, multiplied by the average annual spend per purchaser.