It was 3am and I was on the floor of my Oxford college convulsing with fear. Shakily I dialled the number for the Samaritans and, between sobs, explained I thought I might have strangled my boyfriend. That I was a 19-year-old female while my boyfriend was a one-time rugby player didn’t do anything to assuage my fears, for I was in the grip of a cannabis-induced breakdown triggered by the stress of my first year at university, an emotionally abusive relationship and, crucially, a lot of weed.

It’s a period of my life I’ve tried to forget, but inevitably I thought of that episode again after a top UK psychiatrist, Dr Lade Smith, warned at the weekend that young people risk damaging their brains with high-strength cannabis. According to Smith, president of the Royal College of Psychiatrists, teens who smoke it habitually significantly increase their risk of developing a psychotic illness by the age of 25. “Anyone who smokes cannabis regularly will admit that they’ve had a ‘para’, and what they mean by that is that they’ve had a paranoid fit,” she said. “People laugh about it.” Sadly I know from my own experience it is no laughing matter.

Like many middle-class London teens, I started smoking cannabis when I was about 15. I preferred it to alcohol, which I thought tasted horrible, and weed gave me a pleasant buzz rather than the blackouts and vomiting I associated with alcopops. Also it seemed comparatively safe — it was “natural” and easy to get hold of — and I only ever smoked at parties, never at home or alone. But by the summer after my A-levels I was smoking almost every day, and when I arrived at Oxford there didn’t seem to be any reason to stop. In fact, almost as soon as I unloaded the car and waved goodbye to my parents I made a beeline for Alex, a boy in the year above whom I’d overheard bragging about scoring some cannabis. Suddenly the drug wasn’t just a way to relax, it had become a shortcut to making friends.

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Fuelled by our shared interest in weed, Alex and I fell into a relationship and soon I spent most of my time in the apartment he shared with another student. I was now part of a druggy set, but since it was “just” cannabis — nothing harder (that I was aware of) — it seemed safe. Every evening we’d gather at Alex’s grubby flat, eat junk food (the “munchies” being a common side-effect of smoking weed) and watch films surrounded by a cloud of cannabis smoke. Now I can see it was seedy and depressing, not to mention potentially dangerous, as I spent my evenings with young men who were off their faces. I also gained a lot of weight. Back then, though, with a joint in my hand I felt cool and sophisticated, even enjoyed a (misplaced) sense of superiority to fellow students who drank so much that they were legless by the end of the night. It never occurred to me that I had no idea what was in the weed Alex was procuring.

While I coasted through the first term on an artificial high, after the Christmas break my workload increased dramatically and I realised I’d over-committed to a host of extracurricular activities, including a student play I couldn’t back out of. Meanwhile the relationship with Alex was making me feel suffocated and we were fighting regularly. But still I kept smoking cannabis, unaware it was slowly poisoning my brain.

Within a few weeks of the new term I was having increasingly unsettling thoughts, which mushroomed into a fear that I might hurt someone. Soon these thoughts were accompanied by panic attacks, where the room would tilt, my chest would tighten and my palms would feel clammy. I remember one evening as I walked back from a play rehearsal on the outskirts of the city with a fellow student, I was overcome with terror that I was about to attack her. I almost ran the entire way home.

Worse was to come. One night, after another fight with Alex, I returned to my room and became convinced I had killed him. I texted him but he was asleep and didn’t reply, which I took as confirmation he was dead. I became hysterical and given it was the middle of the night, didn’t know who else to call except the Samaritans. Thank goodness for the steady voice on the other end of the line, which calmly talked me down. Alex was, of course, completely fine, but that episode was a turning point, and I had to admit to my college tutors I was having a personal crisis — although I didn’t tell them the exact details in case I got into trouble. I went home for a couple of weeks to recuperate. I didn’t even tell my parents the entire story, although I think they suspected.

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Even though I haven’t touched cannabis since (and rarely drink alcohol), occasionally I still suffer from distressing intrusive thoughts. There was also a horrifying and tragic postscript to my story. One of Alex’s public school-educated friends, who was a regular at the cannabis-laden gatherings in Alex’s apartment, did go on to murder someone in a frenzied stabbing attack. During the trial, drugs were found to have been a significant factor; he’d been a habitual cannabis user since the age of 13 and had developed schizophrenia.

Which is why I wholeheartedly support Smith’s campaign to counter what she describes as a “very strong” cannabis lobby, and encourage the government to educate people that the drug is not safe, particularly for the young. I just wish I’d heard that back then.