A new diet drug for dogs which can cut their weight by a fifth is no substitute for a regular walks and good food, vets said last night.

Slentrol is billed as a weight loss drug for plump pooches whose owners can't resist giving them fattening treats or simply don't have time to exercise them.

But the Royal Society for the Protection of Cruelty to Animals said pills are not the way to tackle the growing weight problem among Britain's 6.8million pet dogs.

Comment: This is just another way for the pharmaceutical companies to make more money. It would be far healthier for the pets if owners were educated on just how much it harms pets to have excess weight. Rather then going for the quick fix, that, most likely, has unpleasant side effects.

Around a quarter of pet dogs are obese, raising their risk of a host of ailments, including diabetes, heart trouble and joint problems

US-based drug company Pfizer says the solution could lie in Slentrol, a liquid that suppresses a dog's appetite and cuts its absorption of fat.

When mixed into a dog's food every day, it can lead to the animal shedding 18 per cent of its body weight in just six months.

For an overweight Labrador, this would be the equivalent of around a stone, while for a portly Yorkshire terrier, it would equate to just a pound or two.

A spokesman for Pfizer said the drug, which costs less than 50p a day, would be a valuable aid to owners who can't help but hand over treats when faced with 'puppy dog eyes'.

He added that the drug, which can only be prescribed by vets, should form part of an overall weight loss programme which would include advice on diet and exercise and regular check-ups.

Comment: For most dogs, a change in diet and exercise is all that is needed to get rid of the unwanted weight. Why put unneeded chemicals in your pet when all that is needed is a little more discipline for the pet owner?

The marketing of Slentrol follows the launch earlier this year of Yarvitan, the first canine diet drug and of a beef-flavoured antidepressant.

But the RSPCA cautioned that the growing use of 'lifestyle drugs' for dogs could mask the real problems underlying their ill- health.

Comment: The pharmaceutical companies are now trying to open up a whole new market for them to rake in more cash. It has been shown that with humans, a lot of the drugs that are taken actually cause other problems which then required more drugs. This is just the case that the pharmaceutical companies would now like to bring down on our pets.

The charity's chief veterinary advisor, Mark Evans, said: "Instead of covering up weight with a pill it's vital to tackle the real reasons why so many pets are overweight.

"Pet obesity is a serious animal welfare problem and as a nation of so-called animal lovers we have a duty to tackle it now.

"In most cases, a lifestyle based on a healthy reduced calorie diet and an appropriate exercise regime is all that's required to bring about weight loss."

RSPCA senior scientist Dr Jane Cooper said that Slentrol could cause unpleasant side-effects including vomiting and diarrhoea.

She added that the tests needed to bring the drug to market had led to the suffering of many dogs, cats and other animals.

In some cases, the animals were put down after the experiments.

She said: "Owners may see diet drugs as some sort of quick fix but I'd like them to bear in mind all the issues that surround these medicines.

"It is often because they love their animals so much that they give them endless treats - and animal lovers wouldn't like to think that other animals had suffered for weight loss products to be available.

"As far as the RSPCA is concerned, these drugs are not the answer to the nation's pet obesity problem.

"It would be very wrong if people turned to diet drugs rather than getting advice on their pet's nutrition and increasing their exercise levels."

Pfizer said all animal experiments were carried out to according to strict government regulations.

Dogs are not the only animals losing the battle of the bulge.

More than a third of cats are overweight and the number of cases of feline diabetes has risen five-fold in 30 years.

Overweight rabbits and even hamsters are also a common sight in vets' surgeries.