In a sectioned-off room in Milan's plush Four Seasons hotel, Chili Peppers guitarist John Frusciante is explaining his unusual methods of trying to come off heroin. "I's tried it by just smoking pot and drinking," he says with a slight slur. "I tried to quit by taking speed and other stuff, then smoking crack and just taking heroin occasionally. I tried it by shooting coke. I knew I was going to die. This was January and I was positive I would be dead by March if I kept on taking drugs. So I tried it without doing any drugs at all. I thought, 'Let's see what happens in a year. If it still feels like the world is against you and people don't want you, you can go back on drugs and you can die.'"

Five years later, Frusciante has had no reason to go back, and he is certainly loved. The Chilis' gig at the Fila Forum arena is so in demand that there are people selling fake tickets, while Donatella Versace has sent the band a bunch of clothes with a handwritten note, politely requesting that rock's coolest survivors put in an appearance on her catwalks. It's a similar story all over the world, especially in the UK, where last August's By the Way album has barely been out of the top 10 since its release, and has now shifted 8m copies worldwide. It will surely outstrip the 12.5m of its 1999 predecessor, Californication.

In fact the album, hard-rocking yet filled with transcendental beauty borne out of great pain, is a microcosm of their career since a bunch of LA schoolfriends first mixed punk, funk, avant garde, disco and jazz with every stimulant going 20 years ago. Nowadays, the Chilis have never been in better shape. Three quarters of the band - Frusciante, singer Anthony Kiedis, bassist Michael Balzaray aka Flea - have gone through heroin addiction, but aside from drummer Chad Smith's lingering indulgence in smoking and drinking, they are now so healthy that Frusciante worries about the amount of fat in a hotel biscuit.

Behind them lies a litany of ups and downs. This is, after all, the band that has outlasted trends, drugs, madness, seven guitarists, nervous breakdowns, £70,000 of dental surgery (Frusciante's teeth fell out from drug abuse) and one death (of founding guitarist Hillel Slovak, from heroin, in 1988). That's six deaths if you include the times Frusciante has OD-ed and technically passed away.

The Chilis have avoided the traditional rock fate (band take drugs, mess up, make bad albums, clean up, but are creatively spent), and perhaps the clues lie in the once broken but daily recovering figure of John Frusciante. The gifted guitarist plays on the Chilis' best and most successful records. He joined the band for 1989's Mother's Milk and 1991's Blood Sugar Sex Magik, and then he quit, finally returning for Californication. While he was out of the band from 1992 to 1998, the Chilis struggled. They produced only one album, 1995's disappointing One Hot Minute.

Sitting bolt upright in a comfy chair, Frusciante is still only 32, ravaged and recovering but extraordinarily childlike and vulnerable. His long sleeves cover the scars of his addiction, but his words convey the thoughts of someone who is intelligent and sensitive, perhaps too sensitive for his own good.

"I's done a lot of thinking and I wasn't very social," he sighs. "I felt like I was turning my brain inside out to where my subconscious was becoming my conscious. I was understanding things that a man doesn't have a right to understand, about the way people's energies work together and who they are. Why a rock star makes one person happy and another makes you... wanna kill them. I was seeing these things in a way that was... disgusting. Really disgusting. I could deal with it until I saw the depths. At that point, everything that was beautiful to me became ugly. Everything that had previously brought me happiness caused me the hugest sadness. Music. Paintings. People. It was pure depression. [Heroin] caused these things to be beautiful again."

In past interviews, Frusciante has hinted at unspecified "childhood pain", but has never expanded. "It's subconscious childhood pain which you've pushed into your memory and then suddenly it pops out 20 years later and you's a drug addict," he whispers. Can we talk about it, I ask. "Oh, come on."

Frusciante grew up in California. Aged six, his parents divorced and he flitted between living with his Mum in Santa Monica and visiting his father in Florida in the summers. But the kid was happy, so what went wrong? "Nothing... I mean, to have childhood pain you don't need an unhappy childhood. It can be one little moment... or a period of a couple of weeks that ends up growing..." A couple of weeks? "Yeah... or 10 minutes in a couple of weeks that can have a profound effect on the rest of your life." The silence is screaming.

In the foyer, bassist Flea is practising yoga moves. If Frusciante is the band's creative force, Flea is the talisman. His posts on the band's website make both hilarious and moving reading. (December 2002: "I've had a hard on for this whole flight. I haven't slept for the whole fucking thing!") But behind Flea's goofball persona lurks a shy, likable, thoughtful man who can feel "overwhelmed by the world", confesses that touring gets "very lonely" and says he can even now be found at parties "staring at my shoes".

Perhaps the oddest thing Flea tells me is that only now - aged 40 - has he finally been able to "forgive" his parents. "I was raised in a very violent, alcoholic household," he says. "I grew up being terrified of my parents, particularly my father figures. My dad left when I was six, I didn't see him much, and my stepfather was aggressive. It caused a lot of trouble in later life." From the age of 11, Flea was hanging out on street corners until 4am. The Chilis' early years were a party, typified by the band's now notorious performances wearing only socks (on their penises) but increasingly, the party took on a darker hue. "We did drugs from a very young age and it just started to kind of... steamroll."

Everything first hit home with the death of Hillel Slovak. "When Hillel died it was during one of the happiest times of my life," says Flea, prodding at his green tea. "I was married and completely in love and had a baby on the way. I was smoking weed and playing basketball and going home and loving my wife. I felt very connected with a lot of people, but a lot of that was shattered. When Hillel died, I completely hit the deck." The shellshocked bassist veered between periods of abstinence and abuse, which came to an end when he hit the bottom at 31. "I was this incredible burst of wildness, and suddenly I was hacked down," he says. "I got sick. I had chronic fatigue for a year. My system completely collapsed. But I was forced to confront things about myself." Frusciante had replaced Slovak, but was falling apart at around the same time as Flea. High in the Hollywood hills, he virtually barricaded himself in at home on a diet of heroin and hallucinations, often staying up for a week at a time.

Flea admits to still having problems with the notion of being a "rock star" as opposed to a musician. "It's something other people see you as and you have to take it with a grain of salt," says Flea, but even now he has been known to snap when pawed by fans and then feel "terribly disgusted and apologetic afterwards". But - perhaps linked in with whatever happened in his childhood - the biggest burdens have been carried by Frusciante. As a teenager, he followed the Chilis: "Their shows were the most exciting place to be." Then he beat off scores of competitors to become suddenly part of those shows. "I made mistakes when I joined the band," he says, surprisingly. "The way I behaved was so careless and one-dimensional. Thoughtless. I thought, girls, money, drugs..." You were 18 years old, I say. "I was too young, yeah. But I had two years of negligence. When I see pictures of myself back then I just wanna strangle the person." Finally kicking drugs was made easier by the realisation that his old emotional problems were behind him. A year later, he was recording Californication. A big thing, says Frusciante, was "realising that people loved me".

One such person was Anthony Kiedis, the arty-Iggy frontman, who was "very tight" with Frusciante originally, but who excommunicated the guitarist for five years. As a recovering addict in 1992, the last thing he needed around him was another user, and when Frusciante left and got into heroin he felt "doubly betrayed". However, as the band floundered, he slipped back into abuse. When Flea was despatched to ask the recovered Frusciante to rejoin them, Kiedis decided to clean up for good. The bad blood between them melted away with the band's delight that the musical chemistry was intact. "It's like jazz musicians," says Smith, of the Chilis' now uncommon method of creating music by jams and improvisations. "Unspoken musical telepathy."

Smith (who joined with Frusciante in 1988) is the most regular Chili, the epitome of the drummer - solid and dependable when all is falling apart. He suggests his blue-collar roots - "Michigan, smoking pot and drinking, occasionally too much" - helped him avoid the band's worst excesses. He is the only one who heads for the bars and fleshpots after gigs, and although he confesses to occasional "mischief", it's unlikely to be drug-related. If Smith has a problem, it's relationships. He has three children by different mothers, and has never been able to settle down. "I'm the dumper," he sighs. "I fall in love easily, but... I get restless. I'll figure it out one day. Can we talk about something else?"

Much has changed in the Chilis' camp. Tofu and fragrant candles have replaced coke and heroin. Flea illustrates the transformation from the "out of control, obnoxious brats" of their youth to where they are now with a story. When producer Andy Gill (of Gang of Four) did what they considered to be a bad job on their debut album, they shat on his mixing desk. Nowadays, Flea insists, they would "calmly explain". But the idea that the band has mellowed totally is as daft as expecting no further bumps along the way.

In front of 15,000 fans at the Forum, the Chilis look and sound unstoppable. Flea looks as cool as any 40-year-old father flea-hopping in orange underpants ever could. Kiedis smashes his muscular frame around the floor. Smith can't hold down relationships but conducts the audience's applause using just a drumstick and a bass drum. Most poignantly, Frusciante stands stagefront with his eyes closed, lashing out searing solos that seem to come from a very private place. "I's at peace with myself," he said earlier, a man who knows he's blessed with a second chance. "I think that when I was a young, confused and stupid person who actually hadn't lived much, I think I really wanted to be who I am now. In a way I's proud of all my experiences because they'se helped me get here." As he turns to face the band, his face explodes in glee.

Read the original interview and more music reviews in The Guardian!