By Michelle Roberts
BBC News health reporter
Figures show a large rise in patients admitted to hospitals in England due to cannabis, and experts are warning these may be the tip of the iceberg.
Jason blames cannabis for his mental health problems
Two ex-users of the drug describe their experiences.
Jason Pegler, now aged 32, was 14 when he first began smoking cannabis.
It started out as a bit of fun that he would do now and again, but soon he was smoking it most days.
"It made me more withdrawn as a person and paranoid.
"And over the years, it made me want to take more drugs, like ecstasy," Jason recalls.
At the age of 17, Jason was hospitalised for a manic episode and the doctors there diagnosed his bipolar disorder or manic depression.
"Cannabis was definitely the trigger. I came down from the high after two weeks and realized that I was in a mental hospital. It was then that I decided I would never do drugs again."
Jason now works as a publisher of mental health books in London.
"I know from the books I publish and the people I help everyday that cannabis alone causes psychosis, and also schizophrenia. I know of hundreds and hundreds of cases.
"So I'm not surprised more and more are being admitted to hospital."
Hallucinations and blackouts
Jo, a 22-year-old from Cambridge, was 13 when she first tried cannabis.
"I had such a horrible night that I didn't do it again for some time. But then when I was 17 my friends were doing it so I tried it again. And it became a habit.
"The thing with cannabis is, it's only with hindsight that you can see how you fell into it.
"At the time I saw it as a treat. I would have a spliff at 8am before work, another at lunchtime and one as soon as I finished for the day. I could smoke between six and 20 joints a day.
"At the time, it was not the problem it was the cure. It made me feel less anxious and paranoid."
But that changed. Jo said she began smoking more and more cannabis just to get the same effect. She also tried other drugs to counter some of the side effects.
"I would use speed to combat the cannabis and night nurse to help me sleep."
Jo said the cannabis caused big gaps in her memory - some over a period of months. "And I would cough up black gunk because I was smoking so much," she said.
Jo also experienced some acute psychotic episodes.
"They were extremely scary. I used to hallucinate that there were spiders crawling all over me and my bedroom walls were bleeding."
At the age of 20, when she began to hear voices in her head, Jo decided that she needed help and wanted to quit using drugs.
She has been clean for 20 months.
She advised anyone who was having a problem with cannabis to seek help.
"There is help out there. You don't need to suffer in silence."
Jo is not her real name. She wishes to remain anonymous.