Scientists have shown how cannabis may trigger psychotic illnesses such as schizophrenia.
A King's College London team gave healthy volunteers the active ingredient tetrahydrocannabinol (THC).
They then recorded reduced activity in an area of the brain which keeps inappropriate thoughts at bay.
THC levels are thought to have doubled in street cannabis in recent years - at the expense of other ingredients which may have a beneficial effect.
A separate study has shown that one of these ingredients - cannabidiol (CBD) - has the potential to dampen down psychotic symptoms, and could form the basis of new treatments.
The research will be discussed at a conference on the impact of cannabis use to be held at the Institute of Psychiatry at King's College this week.
Although figures are not kept, it is estimated that as many as 500,000 people in the UK may be dependent on cannabis.
Increasing numbers of people are seeking help for cannabis problems at specialist clinics. In 2005, only heroin users accounted for a greater proportion of patients.
Experts are concerned that street cannabis is becoming increasingly potent. It is thought that average THC content has risen from 6% to 12% in recent years.
The Institute of Psychiatry study gave THC, CBD or placebo capsules to adult male volunteers who had not abused cannabis.
They then carried out brain scans, and a battery of tests, and found that those who took THC showed reduced activity in an area of the brain called the inferior frontal cortex, which keeps inappropriate thoughts and behaviour, such as swearing and paranoia in check.
The effects were short-lived, but some people appeared more vulnerable than others.
In a second study, a team from Yale University administered THC intravenously.
Even at relatively low doses, they found 50% of healthy volunteers began to show symptoms of psychosis.
Volunteers who already had a history of psychotic symptoms appeared to be particularly vulnerable.
A third study, by the University of Cologne, compared the effect of CBD and a commonly used anti-psychotic medicine, Amisulpride, on 42 patients with a history of schizophrenia.
After four weeks both groups showed a reduction in psychotic symptoms, but the CBD group were less prone to side effects, such as muscle stiffness and weight gain.
The researchers warned that THC and CBD compete with each other biochemically, so a rise in THC levels would blunt any positive impact of CBD.
Professor Robin Murray, a consultant psychiatrist at the Institute of Psychiatry, said the research provided the strongest evidence that cannabis had a significant impact on the brain.
He said proving a long-term effect was extremely difficult, as it was not ethical or feasible to stimulate long-term psychosis in volunteers.
However, he said: "If something has an active effect in inducing the symptoms of psychosis after one dose, then it would not be at all surprising if repeated use induced the chronic condition."
Professor Murray also warned that the high potency cannabis now widely available was likely to pose a much bigger risk to health than the significantly weaker formulations of previous years.
"It is similar to comparing the effect of drinking a glass of wine at the weekend with drinking a bottle of vodka every day."
Marjorie Wallace, of the mental health charity Sane, called the research a "significant contribution" to the understanding of the dangers of cannabis.
"Sane has been saying for years that there is a link between psychosis and the drug, particularly in its more potent forms.
"We strongly urge the government to heed the growing evidence and take urgent action to warn young people that some of them are risking lifelong mental illness - that they are playing Russian roulette with their minds."