A mental health charity has called for a government inquiry into the effects that cannabis has on users.
Rethink wants the Commons Health Select Committee to launch an investigation “to help establish the facts about the link between cannabis and psychosis”.
It comes a year after the government reclassified cannabis from a Class B to a Class C drug.
Rethink said the reclassification had sent a “confusing message” to young people that cannabis is risk free.
Cannabis was reclassified so that police could target hard drugs.
Rethink said there had been a 60% increase in people who smoked drugs and had mental health problems in the last five years.
Most medical experts agree that smoking cannabis in itself does not cause mental illness, but that people who are predisposed to psychosis are much more likely to develop symptoms if they use the drug regularly.
“Cannabis is not risk free,” Rethink chief executive Cliff Prior said.
“We have known for years that using cannabis makes the symptoms of schizophrenia far worse in people who already have the illness.”
Calling for further research, Mr Prior said the government should “concentrate on the real and specific mental health dangers, not general warnings that no-one takes seriously”.
Home Office figures released on Friday showed that arrests for possession of cannabis fell by a third in the first year of its reclassification.
Based on the feedback from the 26 police forces, there were an estimated 43,750 arrests in the last 12 months compared with 68,625 in the previous year.
In response to the figures, drug education charity DrugScope called for more debate on drug education and prevention, particularly in schools.
“We need to move on from the debate on cannabis reclassification to concentrating on practical responses to drug use and harm,” said chief executive Martin Barnes.