There have been so many Worst Moments. There was the morning we watched our son shamble down the street, spare T-shirt and shoes stuffed into his shoulder bag, his 17th birthday only days before. His mother and I have just told him to leave home, that we cannot accept his behaviour any more. His front-door keys are still lying where he threw them on the doormat. We gaze from the front window until he turns the corner and then we simply stand there, watching the empty street, wondering what to do next.

Or was it worse when, maybe an hour before that, his mother and I realised - without even a word exchanged - that we had both finally reached this tipping point? All the addiction experts and all the drug counselling literature told us that this was the only way - "exclude the addict until he understands and asks for help" - but we had fought against this final step, telling ourselves we could handle anything, he's our son, we would never expel him. But that morning we see afresh the lank, lost squalor in which he is choosing to live, the wilful self-destructiveness, and finally we understand the inevitable flow of cannabis from him to his younger siblings. We have to protect them, we have to protect what remains of home life.

Or was it several weeks before that, when I attended my first Families Anonymous meeting? Quaint church hall in Wandsworth, welcoming and gentle faces, all of them related to drug-users, recovering or otherwise. The secretary turns and asks me, as a new member, if I would like the chance to say anything. I've readied myself for this moment, I know what I have to say. I open my mouth to speak, to say out loud for the first time, "My son is addicted to cannabis." Instead my throat dries, lumps up, the words won't form. I don't want to cry but I can't speak. I am mumbling, staring at the floor, and they know enough to leave me be.

Or maybe it was a year and a half later. After twice returning to live at home and twice being asked to leave again, we have rescued him from sofa-surfing by underwriting the first month's rent on a flat. Just five weeks later - following persistent noise complaints from all the neighbours, following a written complaint from the primary school whose playground backs on to the flat, following a police visit to break up a fight between him and his flatmate - he has been evicted. He simply takes a trip out of town, leaving his mother and me to clear up and, most importantly, to rescue his cat. Kitty was his sixth birthday present and has been hugged nightly and cherished every day since. He missed her so much during his homeless days, we could not refuse his request to take her to this new flat. Now she is cowering bewildered in a corner, her legs crimped tight under her, her eyes gummy and masked. We ease her on to a cushion in the carry-box and take her back home.

Or maybe it was six months after that. He had come round to collect some possessions from our house. He is flippant, off-hand, but I am simply furious. Out of nowhere, I am ablaze. I can't hold it back. I will always forgive him everything but I am still finding it hard to forget the damage he has inflicted on his two younger siblings - the chaos, the anxiety and, ultimately, the drugs. He could do this to himself but not to them.

As he rummages through the scrappy boxes we brought back from the abandoned flat, I pick an argument and I pick it and I pick it and then I simply let go and am throwing a punch at him. Of course, I don't know how to punch someone. He easily knocks me away and we grapple meaninglessly for a few seconds. Inside I have three, four years of frustration wanting to blow.

His mother sees him out of the house while I slump on the sofa and weep, gasping, snotty, desperate, final: "I have lost my son." I have deliberately tried to strike my son, to punch him until he hurts. Who have I become? What happens to make anyone do that? And yet, later that day, we find him sitting in the park outside, strumming a guitar with his sister.

I apologise (as though I could ever say enough). He smiles and says it is OK (and that's why I love him).

And for the last four years, this is how it's been. Two steps forward, two steps back. We effectively remain where we have been since it started.

This is cannabis. It stops you, it rips out normal reactions, normal kindness, normal motivation. It draws a line and you stand patiently behind it. And this is why we have broken one of the most serious prohibitions facing any writer. You Do Not Write About Your Children. Yes, your kids might enter your work now and then in charming disguise but you do not ever lay out their genuine, raw problems on the page. You fictionalise them, you do not present it up-front and true. There is a glass-fronted box in the corner of every writer's room, protecting the real lives of their children: Smash Only In Case Of Emergency.

This is an emergency. True, the city is not aflame, plague is not afoot. But there are too many families whose home life has been shattered by a teenage son (it is nearly always boys) who is losing it as a result of cannabis. Maybe not as badly as ours has lost it, but nevertheless creating chaos and distress. We think our boy was one in a thousand, maybe one in five hundred. He drew the unlucky lottery ticket, his brain could not cope with this influx of chemicals.

The Department of Health figures (one in five will have tried cannabis by the age of 15) insist use is falling in Britain. That's not how it feels in south London - or, presumably, south Manchester or south Glasgow. Here, it feels like everyone has had a toke by the age of 15. Of these, only some will become regular users. For most of them, it is a Saturday-night high and nothing more. But for some, it becomes unshakeable.

And crucially, with this particular drug, this is happening to children - to 13, 14, 15 year-olds. So if anyone is going to write the inside story, to bring out the truth of this, it is going to be a parent. My wife found herself doing this - long after the worst of the grief - but when the book was finished the decision was mine. I told her it was only publishable if our boy agreed. Over lunches in our local Italian, she showed him the manuscript and, subject to a few factual corrections, he agreed. And later, when we happened to find some poems of his, he selected the ones he was happy to see included in the book.

I know there are those who will say that he had no real choice, that he understands what makes his mother write and knew this book was precious to her, important even. But I also know that if he had thrown it back at her, horrified, she would have instantly withdrawn the manuscript. It is madness to suggest that she would put a book ahead of her relationship with her son - or that I would let her.

In my optimistic moments, I even imagine that he knows and understands the mess he is in and knows the story is true and deserves to be told. Maybe not. I just can't stop myself hoping. Every day I wait for him to come back. Every day.

Imagine if you could wave a wand and instantly all the spliffs and baggies were transformed into bottles of gin. You leave for work on Wednesday morning and suddenly you see kids on the way to school with a quarter of Gordon's sticking out their rucksack; at Thursday lunchtime, you see them sharing a swig of Tanqueray at the bus stop. And if you saw that daily, all around you, you would say there's a genuine problem. Except it's worse than that. Because skunk gets you as high as gin but has psychotropic effects to boot. Cannabis remains in the bloodstream for up to 10 days and, let me tell you, the mood swings continue for every one of those days. And that's not all. In your early 20s, the legacy returns in the form of schizophrenia. Professor Robin Murray at the Maudsley Hospital estimates that at least 10% of all people with schizophrenia in the UK would not have developed the illness if they had not smoked cannabis. That's 25,000 individuals at current figures. With stronger varieties being smoked at a younger age, this figure can only rise. So tell me, Daily Mail, why are you treating this story like "a bit of pot"? Why focus on the blonde novelist when there's a much bigger issue here?

Looking back, our boy seems to have started losing his way in the months leading up to GCSEs. He had been the star pupil, the star boy, our eldest, our golden one. Even as a child he had easily mastered the knack of charming adults. He was easygoing and biddable, with a genuine smile and ceaseless energy. We asked him to work hard at school and so he did. In his first year at secondary school, he wangled himself on to the headmaster's table at the annual quiz night and they won - he loved it.

By the time he was 15, he was targeting 12 A-grades at GCSE. A year later, we were so exasperated with his behaviour I remember saying to Julie, angrily, desperately, "He needs to fail one of these GCSEs. He needs to realise what he's doing."

Of course, it was us who didn't realise. By then he was smoking cannabis, presumably the potent form of cannabis known as skunk. In a Home Office study, figures from 23 police forces suggested that 81% of cannabis seized last year was herbal cannabis and the majority of this would have been the stronger form known as skunk. Sometimes he'd admit to using skunk, sometimes he'd insist he wasn't. Sometimes he'd tell us he used it every day, sometimes he'd boast about not having touched it for three days. Drug users are rarely consistent. But it was certainly draining all motivation out of him.

Over the last few days, most of the British press has queued up to criticise Julie for writing about the devastation that skunk has worked on our family. Their arguments - some ill-informed, some plain vitriolic - have all rested on an implicit belief that "a bit of pot" simply does not cause this kind of aggression, this sort of abuse. Yes, they say, if this was a heroin addict, nicking your stereo, your jewellery and flogging it down the pub, that would be credible. And they're right, you don't need to flog a stereo for a spliff - it costs less than a pint. And anyway, cannabis makes you mellow - stoners are hippies, laid back, docile to a fault. We used to smoke it, they imply, and we just giggled.

That was then. Skunk is GM cannabis. Evidence from the Forensic Science Service suggests that skunk cannabis (otherwise known as sinsemilla) is remarkably stronger than ever before. It is unquestionably different, definitely stronger. In skunk, the active ingredient, THC (tetrahydrocannabinol), has been ramped up significantly. But perhaps more importantly, this has been achieved at the cost of another component of naturally occurring cannabis, CBD (cannabidiol). And some scientists are starting to think that CBD has antipsychotic properties - something to offset the THC in old-fashioned marijuana but absent in skunk. But hang on, says the commentariat, you don't see stoners getting violent, abusive. You just don't. And, I agree, anecdotally that feels true. But these are adults they're talking about (and most of them have dropped out, are not being told to get up and go to school at 8am every morning). What happens if you give this potent, psychoactive ingredient to young, still-forming brains? There probably isn't enough certain scientific evidence yet (how long did it take for Richard Doll to gain a following for his cranky smoking-causes-lung-cancer theories?) but the anecdotal evidence is colossal, alarming, unavoidable.

Over the last three years, we have started to mention to friends what has been happening to us, the days and weeks of abuse and chaos. Too many of them have said, "That's amazing, exactly the same happened to my cousin's boy," or, "Yes, I know, my neighbour lost two sons to cannabis."

It's all just teenage rebellion, the doubters presume. They tell us we have overreacted. (Our son tells us the same, though he also readily admits to persistent drug abuse.) And for a year or so, that's how we interpreted it. When he stayed up all night and slept all day, when he stole regularly from us, when he returned home at 3am and woke his brother or sister for a chat, when he kicked open locked doors, when he insisted on coming to Sunday lunch in just boxers and picked an abusive argument when we asked him not to. In fact, he picked an argument about almost anything, almost daily. If we tried to deny him money, he stood belligerently in front of his mother's desk, refusing to let her work. Day after day, boundaries were ignored, order reduced to disorder. And maybe the worst thing was that his siblings were starting to change, they were echoing his disregard, his abusiveness, presuming this was correct pre-adult behaviour - of course they did, he's charismatic, he's the older brother. The whole tenor of home life was sliding, we would retreat to a boundary to create peace and he would march straight up and smash through that.

Any single one of these instances would pass for teen spirit. Put them all together and there's something different happening. Even then we didn't assemble the picture until an old friend who now lives in New York came to stay one night. We told her what had been happening. She didn't think twice, she told us it was drugs. We said, sure, we know he probably smokes some dope, that's all part of the rebellion. No, she told us, the cannabis isn't a symptom, it's the cause. That's when we put it together. A week later I was sitting in a FamAnon meeting. Two weeks later I knew all about Tough Love. I watched other FamAnon members retell the pain of walking away from their children, knowing it was the only way, the only hope.

It was a horrible learning curve, at once both a simplifying relief - we could finally put a name to this persistent, chaotic barrage - and an appalling admission. By my second meeting, I was able to say, out loud, "My son is addicted to cannabis." His two grandmothers both still reel at our use of the word "addict" but what else do you call it when his life has come entirely derailed? When he tells us what he really wants to do, what he passionately cares about but never manages to do it?

He is an outstandingly talented writer and all he wants is to make it as a songwriter. I think he will. But right now his life has stopped. He started an evening course that might have got him into university to read English - he dropped out of school after two years spent scraping three AS levels. But then he failed to show up for the end-of-year exam. Things start and never get completed. He is 20 now and has never done a day's paid work in his life.

Again, name any of these instances and they are hardly proof of drug problems. Put them together, and put them in the context of the happy and fulfilled boy he used to be, and you start to see the insidious effect of cannabis addiction. Simply because he is not begging on a street corner (except when he's busking, which he does with glorious chutzpah) or drooling with a spent needle hanging from his arm, you presume he is doing fine. And if you met him now, you would meet a tall, healthy-looking, articulate, charming guy. Look deeper and you would see a life in stasis.

Should we have allowed him to remain living here? The sensational press and ill-informed columnists have painted us as a couple who found one little spliff and told him to pack his bags that afternoon. We fought for almost two years to avoid doing that. And even after the first eviction, we took him back, renegotiated, watched him bust through boundaries, heard our other two children beg for peace. I remember one evening, late autumn, between evictions, he returned home and within 15 minutes I found my wife, my daughter and my son each in separate rooms, all in tears or shock. Our hearts sank each time we heard him re-enter the house.

During all this, we visited a drugs counselling specialist - and there are not many in Britain who know enough about cannabis - who comforted us but laid it on the line: there would be no other way out in the end. We attended FamAnon and discovered that there is a frightening tendency for this habit to pass from elder to younger brothers. That was a chilling harbinger. We had a 14- and 13-year-old to protect.

And so, unable to change, he went. The family balance was destroyed. In truth, we have never recovered from this, our family home will never be the same again.

Ask any family that has been hit by drugs and the first word they will use is "lonely". When this happens to you, no one who has not experienced it can or will understand it. And when it happens to your pre-adult child, it is doubly incomprehensible.

So if Julie's book helps any other family identify their problem sooner, get help sooner, steer their son off this path, then I don't mind what anyone says about her or me. We don't mind what they think of us for publishing a book about our own son. Our relationship with him is precious, enduring but ultimately our problem. Your problem starts when your child smokes his first skunk. And maybe then you'll pick up her book and want to understand.