The obesity pill, orlistat, has been licensed to be sold over-the-counter at chemists in the EU.
The pill, which works by blocking the absorption of fat in the body, will be available at a lower dose than doctors generally prescribe.
It is aimed at adults with a Body Mass Index (BMI) of 28 or more.
One expert said it may help some people with weight loss but they would have to take a conscious decision to eat less fatty foods.
The Royal College of GPs said everyone who wanted the pill should go to their GP for advice because some of them would have treatable causes for their obesity - and all of them would have risk factors for other illnesses which would need to be discussed.
GlaxoSmithKline, who make the pill known commercially as alli, say their clinical trials show that adding orlistat to a reduced calorie, lower fat diet, can help people lose 50% more weight than dieting alone.
But if they persist in eating fat, they will experience some nasty side effects such as diarrhoea and gas problems.
Training for pharmacists
Community pharmacist, Graham Phillips, who advises the National Obesity Forum, said he saw no problem with introducing the pill over-the-counter.
"There will be a training pack for pharmacists which will teach them how to measure BMI quickly. Waist measurement is 90% of what you need to know."
"Pharmacists have been dispensing emergency contraception over-the-counter for some time, so they have systems in place like consultation rooms to make sure these pills can be dispensed safely to the correct people."
Dr Ian Campbell, medical director of the charity Weight Concern, said: "It can work, the risks are minimal, but I don't want to see the public being exploited. They have got to use it with knowledge.
"This plays a part in the a weight management regime - but lifestyle changes in terms of eating a better diet must come first and last."
But Professor Alan Maryon-Davis, president of the Faculty of Public Health, said: "My worry is that a lot of people will take these pills without proper advice and support about dieting and exercise."
"It's all too easy to pop a pill instead of making the lifestyle changes we need to keep our weight under control.
"This will only work if pharmacists can spend a lot of time giving ongoing advice."
And Steve Field of the Royal College of GPs said: "The side effects of this drug are very unpleasant in many people. The pill should only be used with support and dietary advice."