A man and woman who were found dead at the weekend had earlier told friends they had taken the so-called 'party drug' mephedrone.

Police yesterday confirmed the 59-year-old woman found dead in Leicester and the 32-year-old man, whose body was discovered in a house in Melton, knew each other.

Friends have told officers the pair had told them they had stayed up all night on Thursday taking mephedrone – a banned former 'legal high'.

However, police believe it is also possible the pair may instead have taken methoxetamine, a variant of the horse tranquilliser ketamine.

Officers are to check the pair's computers to see if they had bought drugs on the internet.

No traces of mephedrone were found at either house, but officers did find some methoxetamine.

Police and drug treatment experts have urged people who have recently bought either drug not to take them and to contact the authorities immediately.

Mephedrone, which is also known as MCAT or meow meow, was made illegal when it was classified as a class B drug in 2010. Until then, it had been the most prominent of the 'legal highs', which were said to be increasingly popular with young people.

Its side effects are said to include psychosis, palpitations, burning throat, nose bleeds and insomnia.

It is particularly dangerous if mixed with alcohol or other illegal substances.

Drug advisers said ketamine and its variants were used for animal surgery and were unfit for human consumption. Side effects include muscle paralysis and a short 'high'.

Indy Thoor, team manager at Leicester's criminal justice drugs team, said: "Ketamine is effectively an anaesthetic used in animal surgery and is not for human consumption.

"Mephedrone is the same.

"Both substances are high risk but they become even more so if they are mixed with other drugs or, particularly, alcohol.

"There is also a danger in buying any of these substances over the internet because people simply do not know what they are buying."

The 59-year-old woman's body was found in an address in Stoneygate, Leicester, on Saturday morning.

Police went to the house when her family became concerned they could not contact her.

When officers became aware of the link with the 32-year-old, they visited the address in Melton 24 hours later.

Detective Inspector Rob Widdowson, said: "One of our lines of inquiry is that prior to their deaths both people may have taken a drug known medically as methoxetamine, which is a form of the drug ketamine and may have been purchased over the internet.

"However, we believe that both people believed that they were taking a drug which is referred to as MCAT or mephedrone. As a result, we are issuing a warning."

Anyone with information about the circulation of methoxetamine or mephedrone is asked to contact Leicestershire police on 101.