WHEN describing what it's like to be a drunk, Sarah Hepola doesn't hold back.

In a typical scenario, she comes out of a blackout on a bed, on top of a guy she's never seen before. "I seem to be enjoying it. I'm making all the right noises," she writes. "My body completes its erotic pantomime. Then, gradually, painfully: 'Excuse me, but who are you, and why are we f***ing?'"

Or, again out of her box, she moons other drivers from the back of her friends' car while stuck in a traffic jam in broad daylight. And doesn't remember anything even when they tell her about it. But there's worse, like when in Paris on a journalistic assignment, she's sitting on the floor of a hotel corridor at 5am, hungover and weeping at the realisation she's left her bag with her passport and all her money in the numberless room of a man she'd picked up the night before and whose name she didn't even know.

Habitual - and deliberate - loss of control through drink is the terrifying leitmotif of the American journalist's life up to the age of 35. Regular blackouts mean large chunks of her memory have disappeared and that loss haunts her to this day. Now 40 and living in Dallas, she's been sober for five years, three of which she has spent writing her memoir in a bid to remember. In its second half she describes the plodding, black, hateful first steps to recovery and beyond. It's this I really want to discuss, partly because it's so rare to have it so well articulated, and because it's something we surely all need to know.

Hepola jokes that her book might sound like a satire of memoir, because she writes about events she can't remember. "But I remember so much about those blackouts, and they haunt me still. The blackouts showed me how powerless I had become. The nights I can't remember are the nights I can never forget."

So we talk across the airwaves between Scotland and America. And we both end up in tears. Tears for the damage done across the world by the demon drink.

Sarah Hepola was born in a quaint Philadelphia suburb where she lived with her parents and brother until she was three. Then the family moved to a busy area of Dallas, Texas, when her father became an engineer for DuPont Chemical in 1970. The move traumatised the family. She says her parents weren't big drinkers back then, "but thirst ran in our bloodlines": her father was a self-conscious Finn and her mother Irish.

Hepola was six when she tasted her father's beer. That first sip "lit a fuse in me that burned for decades". Shy and self-conscious, she became addicted to the liberating effects of Pearl Light beer, and first got really drunk aged just 12. This was when she had her first blackout.

She spent her entire youth and early career as a journalist stumbling from one dangerous situation to another, always emerging with a hangover and the same nagging question: how did I get here? This wasn't just the occasional end-of-the-day tipple; it was a hurtle to oblivion on a regular basis. Her behaviour continued long after her friends grew out of it and - to her chagrin - she became alienated from them.

"I would listen in disbelief as they told stories about me that were like the work of an evil twin," she says. "I said what? I did what? These are the times when you want to die. Friends would say, 'We need to talk. Do you remember what you did last night?' There's a certain point when you fall down the staircase and look around and no-one is amused any more." Although a voice in her head had been telling her she was in trouble from the age of 20, she preferred denial. By 35 she was in a sorry state: three stones overweight with stomach ulcers, a mysterious all-over rash and twisted knees so painful she could not manage stairs.

Her story is scary, but it's one that will surely resonate with other modern young women in the US and the UK, for whom drinking to excess is a regular habit. Years before she quit, the journalist in her set out to know what blackout actually is, and learned that the mechanics are quite simple: the blood reaches a certain alcohol saturation point and shuts down the hippocampus - the part of the brain responsible for making long-term memories. "You drink enough, and the beast stops twitching. Shutdown. No more memories." It's worse for women, especially those like her who are naturally small and have a dangerous habit of skipping meals.

Meanwhile, the short-term memory still works, but short-term memory lasts less than two minutes, which explains why wasted people can follow a conversation from point to point, but they will repeat themselves after some time has passed, what a friend of hers calls "getting caught in the drunkard's loop": the tendency to repeat what you just said. People in a blackout can be surprisingly functional. "They can talk and laugh and charm people with funny stories of their past. They can sing on a karaoke stage. They can run their greedy hands over a man whose name they never asked." The next day, the brain will have no imprint of these activities, almost as though they didn't happen: information that wasn't stored in the first place cannot be retrieved. The National Institute on Alcohol Abuse and Alcoholism states that it's not the type of drink you consume that causes blackout; it's the amount of alcohol you put into your bloodstream, and how quickly you get to that level. Fragmentary blackouts start at a blood-alcohol content around .20, while en bloc blackouts start at around .30.

Hepola's contact at the NIAAA says that if they were selling a drug at the petrol station that shut down areas of the brain so that you were functioning with amnesia, we wouldn't have it. "A blackout is like early Alzheimer's," he says.

If you're reading this after a Saturday night on the tiles, it's possible you find it rather amusing. After all, Hepola's is an American problem: in the post-Prohibition, post-feminist US, half of all women drink, and by 2013 binge drinking had become a "dangerous health problem" for women aged 18-34, with nearly 14 million women in the USA enjoying an average of three binges a month, according to a Centers for Disease Control and Prevention report cited by Hepola.

But it's also a problem in Scotland, where alcohol is now 60% more affordable in off-sales than it was in 1980, over-consumption causes around 20 deaths each week. Alcohol-related hospital admissions have quadrupled since the early 1980s and in 2010, liver cirrhosis was around 40% higher in Scotland than the EU average. Women are particularly at risk: a recent Global Drugs survey showed that over half of British women drink too much.

Was Hepola simply a child of her time? "In the 1980s drinking was known as 'female empowerment'," she says. "By the late 2000s, bumbling blotto heroines were a staple of our narratives. In Bridget Jones' Diary, Sex And The City and others, girls drank and were cool. When I was at college, we felt there was something untrustworthy about people who crossed their arms at the bacchanal."

But she wonders whether she has a genetic pre-disposition to alcohol. "I'm hard-wired to be a drinker," she says. "I have an Irish mother and a Finnish father: two of the hardest-drinking cultures mashed together. Therapists say that alcohol is either a depressant or a stimulant. The people who have a drinking problem are those for whom it creates a spark. I spoke of alcohol in pyrotechnic terms. It fired me up."

Does she think there's a conspiracy within the drinks industry to cover up the truth about the dangers of alcohol? "I hesitate to use that word," she begins, "but there is so much money to be made there. For so long it was the tobacco industry that glamorised products that had such a huge cost to society. I think alcohol is also like that. According to the OECD, alcohol now accounts for a higher proportion of deaths worldwide than HIV, Aids, violence and TB combined."

She describes the process of deciding to quit for good. It was the result of "an accumulation, not by a single act, but a thousand". Friends had given her final calls on their friendships. She was in the bath at her tiny Manhattan apartment , and a slow realisation took grip that she would carry on like this, a hopeless little lush who had let go of herself. "I don't know how to describe the blueness that overtook me," she says. "It was not a wish for suicide. It was an airless sensation that I was already dead. The lifeblood had drained out of me." Before she abandoned all hope, she rang her mother and told her she was going to quit.

She took to sleeping in the tiny cupboard in her hallway, because it made her feel she was being held. She didn't want to return to AA, where she'd been on and off for years, because it made her feel "old and ordinary". She wanted to try it on her own, cold turkey, and she did for three miserable weeks before reluctantly returning to AA'. She was in a "black cloud, a storm cloud, with every day bringing new misery into focus". She swapped binge-drinking for binge-eating.

"I'd spent years losing time, nights gone in a finger-snap, but now I found myself with way too much time," she writes. "I needed to catapult into a sunnier future or slink back to a familiar past, but what I could not bear was the slow and aching present."

She has to re-learn how to live in the moment and not rely on drink to get her through. She has many difficult conversations with friends in a bid to repair friendships, make amends. But she manages it.

The process of repair is clearly ongoing. Each time our Skype session cuts out, Hepola's face is frozen in an open-mouthed expression I can only describe as agonised, like the ghost of Munch's The Scream. It's unsettling; she says she still feels sorrow for the hurt and damage caused over the years.

"Damage to my own life, OK, but when I think about how it hurt my friends and how I let them down, it chokes my throat, because the thing I prided myself on was being a good girlfriend. Girlfriends are so careful and loving with each other, and when I stopped doing that, that was devastating," she says. "Problem drinkers turn into takers."

She concedes it's not over yet, that the temptations are still there. "I'm still walking out of the woods. I don't think I'll ever be fixed. I have daily struggles with self-consciousness, anxiety, insecurity. I'm reminded daily, 'This is why you drank'. I have to find other things to do. I just keep trying to go where the water is warm, to be with people who give me a good hit, to have powerful conversations, my feet in the sand.

"The movies cheat you. They work on the idea of epiphany but in life it never works that way.

"People ask me about that moment I decided to quit, the moment I knew I had a problem. I knew at age 13. I had epiphanies every day. One day I did quit, five years ago." Even after the nadir of the Paris experience it took another few years to make the decision. She weeps again as she says this, but she insists they are also tears of happiness.

Her fragility is palpable, hope so tentative. That she's managed it thus far, and retained her old friendships, is surely testament to her character.

Yet she fretted that people would stop reading at this point. "Books about recovery often end when the heroine gets sober. Would people stop reading when I stopped? But I did feel there was an opportunity to write about how you get out of the drinking problem."

The book is dedicated "to anyone who needs it". Was her motivation to help other women?

"There's nothing I can tell them to do. I am hardly in a position to tell young women to stop drinking, but I'd ask them, 'What are you drinking to avoid?' That's a good question to ask.

"I hear other women say they don't have a drink problem, but they know drinking is masking something in their lives. It's to avoid something."

Being back in AA has pointed her down a spiritual path. When she listened to someone else's story in meetings, she was lifted out of her own sadness and the connection between them felt like a supernatural force she could not explain. "I needed to be reminded I was not alone," she writes. "I needed to be reminded that I was not in charge.

"That's not something I ever wanted, not at all. But the further I go into sobriety that more I realise I have been craving a higher being all along."

Her spiritual life is in its infancy, but the major epiphany for her was that she needed one. "Humans are born with a God-shaped hole, a yearning, a hunger to be complete. We get to choose how we fill that hole." Quoting David Foster Wallace, she says that "pretty much anything else you worship will eat you alive".

She kneels every morning and "bows to the mystery of all I don't know", and says thank you.

"How could I not say thank you?" she asks. "I'm here, I'm 40, I've written the book, I have good relationships with wonderful friends. There are so many ways it might not have turned out like this."

And she knows more than most that's something not every alcoholic gets the chance to say.

Blackout by Sarah Hepola is published by Two Roads, £12.99.