Nic was a lovely child, though of course I’m prejudiced. I’m his father. According to the external barometers we often use to measure how our kids are doing, as Nic grew up – in California where we live – he was doing great. He was a good student, had good friends and his teachers described him as a leader. Nic did have to endure the trauma of his mother and my divorce (he was four), but he seemed to weather it well. Most important in my mind, he was kind, loving and moral.

I found a bag of marijuana in Nic’s backpack when he was 11. It’s not that I was naive about drugs – I knew about the ubiquity and temptation (when I was a teenager I used many) – but Nic was so young. It broke my heart.

What was going on? What I now know is that though on the outside it appeared that Nic was doing well, he suffered inside, plagued by self-doubt, anxiety and depression.

Nic’s drug use escalated and, by the time he was 17, he’d tried pretty much every one you can name. He now says that everything changed for him when a friend gave him crystal methamphetamine. He didn’t know it was possible to feel so alive. No wonder he went back for more. And more. Nic became addicted, and his life – and mine – went into freefall.

He became unrecognisable. He broke into our home, stole from us and even from his beloved little brother, Jasper. Every time I thought it couldn’t get worse, it did. Nic was arrested. Once, an ER doctor called to say he might have to amputate Nic’s arm, which had become infected because of IV drug use. He overdosed more than once. Another ER doctor called one predawn and said: “Mr Sheff, you’d better get down here. We don’t know if your son’s going to make it.”

Nic’s drug use would be interrupted by catastrophes – overdoses and arrests – and I was able to get him into rehab programmes. Each time I felt hopeful, but he quickly relapsed after being discharged. This cycle continued for an interminable decade until, after half a dozen treatment programmes, Nic stayed sober for a year and a half.

I’d been caught off guard when he became addicted. Like most people I knew, I thought drug addicts were the kinds of people we see in doorways in neighbourhoods most of us try to avoid – people obviously strung out, often homeless and possibly psychotic. I didn’t think my son could become addicted, but he had.

Addiction comes with shame and guilt... I suffered in silence

I wanted to forewarn other parents, so they’d know, yes, it can happen to you. No family is immune. I wanted to plead with them not to deny what’s in front of their eyes and waste no time in intervening.

To spread the word, I wrote about our family’s experience in an article in the New York Times Magazine. When it hit the newsstands, I received an outpouring of letters from people who’d been or were then facing their own or a loved one’s addiction. Like I’d done, they’d kept it a secret, often from their closest family and friends, bcause they didn’t want to be judged.

If a child had another disease, we’d be open about what we were going through, but addiction is stigmatised and comes with shame and guilt. I suffered in silence like so many of those who wrote and shared their stories. Until then I didn’t know how ubiquitous addiction is and I decided to expand the Times article into a book, which I called Beautiful Boy. The title came from a song I sang to Nic when he was little.

In the article, I wrote that Nic had always been a good writer. At 16, he won an award for high-school journalists for his school newspaper column. An editor of young adult books who read my article contacted Nic to ask if he might be interested in writing about his addiction. Nic was thrilled. His book, Tweak, was published the same month as mine.

That was 10 years ago. Since then Nic relapsed twice, but he’s been sober for eight years. He is now 36.

Soon after the books were published, Nic and I were contacted by a movie producer asking if the film rights were available. We were apprehensive to turn over our life stories to others who would have carte blanche to portray our family in whatever ways they chose – we’d have no approval. In addition, a film would open old wounds.

We agreed to go forward for two reasons. First, we were impressed by the producer’s commitment to tell a story that avoided Hollywood clichés and stereotypes about addiction. Second, we saw a movie as a way to reach more people who are suffering – to affirm their experience, offer comfort and show they aren’t alone.

The filmmakers cast a then-unknown actor named Timothée Chalamet as Nic and – beyond surreally – Steve Carell as me.

The movie was nearly completed by last September and we were invited to a private screening of a rough cut. I attended with my wife, Karen, and son Jasper, who happened to be visiting. I knew the story, of course, but reliving it was devastating.

Since his breakout role in Call Me By Your Name, Chalamet has been described as a young Leonardo DeCaprio and James Dean. Not only does Chalamet resemble my son when he was younger, but he embodies his spirit. I felt as if I was watching a home movie.

I’d read Nic’s brutally honest accounts of his drug use, but watching it sickened me. The hardest scene to sit through recreated a time Nic was living on the streets, when he ran into a girl he knew. She’d been addicted, but had never injected drugs. I knew what happened next and I braced myself. Nic scored meth, shot her up and she overdosed. When Nic realised that she was unconscious, he performed CPR and called 911.

In the vacant theatre I trembled. It was the first time I’d faced the reality that it wasn’t only a miracle that Nic survived his addiction. We could have had to live with the knowledge that he’d killed someone, too.

Watching Chalamet broke my heart. It was shattering to watch Carell. I was looking into a mirror. In Carell’s eyes I saw my eyes –my pain, desperation, anger and terror. I saw my heart break. Karen, Jasper and I left the screening, got in the car, and no one spoke. Finally Jasper said all there was to say: “We are so lucky.”

I’ve watched the movie one more time since, at its premiere at the Toronto film festival. In the darkened theatre, this time with an audience of 3,000, the story unfolded. Tears came again, but at least I wasn’t caught off guard. I sat a few rows behind Carell and we found each other afterwards and embraced. He’d been crying, too.

People say the movie validates their experience and they feel less alone

Outside I was met by a line of people who wanted to talk. Their emotions were palpable. Some could barely speak because they were sobbing. They said versions of the same thing: “You told my story.”

In person and in messages on social media, people have said similar things again and again and again. Many said they were grateful for the portrayal of addiction as a disease, not a choice made by selfish people. More people than you can imagine told me that their stories were similar, but had a different ending. Their beautiful boy or girl didn’t make it.

People tell of their heartbreak, but also of hope and healing. They say the movie validates their experience and they feel less alone. Some have written to say that it caused them to face their own or a loved one’s problem for the first time. A teenager wrote that she was addicted to pain pills and was suicidal, but no one – not even her family and friends – knew. After the movie, she went home and told her parents. She was afraid they’d yell and punish her, but instead they cried with her, embraced her and asked how they could help. She was beginning treatment.

People wrote that the movie led to reconciliations in families shattered by addiction. A boy wrote: “I just got out of detox and went to the movie with my dad. In the dark theatre I suddenly realised he was holding my hand. I have never been so grateful to cry so much.”

My fear of agreeing to allow others to make a movie that would expose our family has turned to gratitude. The notes that flood in remind me that almost everyone is suffering, whether with addiction or other challenges. There are no easy answers, but isolation and silence make problems worse. Though sharing our stories isn’t a panacea, we are reminded we aren’t alone. And we aren’t. We can be supported. Healing can begin.

Beautiful Boy is released in cinemas on 18 January