A new study says that smokers, who take the highly potent form of cannabis known as skunk, have been found to be almost seven times more likely to develop a psychotic illness than those who use the traditional strength drug.
The first of its kind study by scientists at the Institute of Psychiatry at King's College London came up with the discovery after analysing admissions to hospital for psychotic illnesses such as schizophrenia, paranoia and serious depression.
They found that the patients were seven times more likely to have smoked skunk than ordinary cannabis or hash.
And this was above the increased risk associated with heavy use of the weaker variety of the drug.
"Our study is the first to demonstrate that the risk of psychosis is much greater among people who are frequent cannabis users, especially among those using skunk, rather than occasional users of traditional hash," the Telegraph quoted Dr Marta Di Forti as saying.
"Unfortunately, skunk is displacing traditional cannabis preparations in many countries, and the availability of skunk on the UK "street market" has steadily increased over the past six years.
"Public education about the risks of heavy use of high potency cannabis is vital," she added.
Previous studies have established a direct link between cannabis and psychosis and it is believed that regular use doubles your chances of developing mental health problems.
Teenage cannabis users are particularly susceptible and have been shown to be more likely to develop paranoid personalities, hallucinatations and even schizophrenia later in life.
Scientists believe that the active ingredient THC - or Delta-9-tetrahydrocannabinol - is to blame for the effect.
Old fashioned hash or marijuana contains four per cent THC compared with up to 20 per cent in skunk, a hybrid form of the plant first developed in the Netherlands.
Today's skunk cannabis also contains virtually no traces of another chemical, called CBD (cannabidiol), which appears to counteract the damaging effects of THC in traditional varieties.
In the new study, researchers collected information from 280 people attending South London and Maudsley NHS Foundation Trust suffering their first symptoms of psychosis.
They found the patients were twice as likely to have used cannabis for more than five years, and six times more likely to use it everyday than a control group of healthy individuals.
Moreover, among those who had used cannabis, patients with psychosis were almost seven times more likely to use skunk.
"Psychosis was associated with more frequent and longer use of cannabis. Our most striking finding is that patients with a first episode of psychosis preferentially used high-potency cannabis preparations of the skunk variety," she said.
The study has been published in the British Journal of Psychiatry.