Jason Pierce – AKA J Spaceman – has a reputation as one of rock's primary cosmonauts. His first band, Spacemen 3, had a motto: "Taking drugs to make music to take drugs to." Spiritualized – whose various lineups he has fronted for the past 22 years – have made seven albums awash with druggy imagery, including the 1997 landmark Ladies and Gentlemen We Are Floating In Space, which came packaged as a giant pill. Within minutes of our meeting, he's discussing his recent intake: an injection each week, pills every day. However, he hasn't been sailing off into outer space on mind-bending narcotics, but taking life-saving medicines.

"I found out I had long-term liver disease," he reveals over nothing stronger than two pots of tea, explaining that, when he followed some friends in having tests to see if they could carry on "abusing their bodies", the news wasn't good. "My liver was pretty gone, basically."

Because conventional ("pretty horrific") treatment would have laid him out for a year, the Spaceman opted to test new, unknown drugs (originally given to leukaemia patients, until doctors noticed astonishing side effects on livers). This meant that, even though he was housebound for almost the whole period, he could at least attempt to make some music. As he puts it, with a chuckle that wryly nods to his old Spacemen motto: "I decided to make a record on these drugs."

The thing is, this wasn't the first time a major scare had served as a creative tool. In 2008 the drily titled Songs In A&E followed an experience three years earlier when he was hospitalised with double pneumonia. His lungs filled with liquid and his heart stopped twice. However, he insists that experience – which included waking up in intensive care to find his children at his bedside, and his partner being given bereavement counselling – wasn't as scary, because he wasn't aware of it.

"It's hard to be aware of your own demise, especially when your body clings on to life," he says, more matter-of-fact than dramatic or reflective. "This time, although the treatment wiped me out, I was aware of everything."

Ironically, Songs In A&E was mostly recorded before his near-death experience, making it eerily prescient. This time, although Pierce had recorded an Icelandic pop strings section in Reykjavik, and backing singers in Los Angeles, he made the album at home during treatment, not telling the "very accommodating" engineer that he was very ill until the end, because he didn't want anybody fussing. "I think he thought I was fucked up," he chuckles, dryly acknowledging his rep. "Sometimes I'd disappear and he'd find me four hours later, asleep in a room, but we got it done."

Today, after being given the all-clear, the youthful 46-year-old is chatty and mischievous with a hint of vulnerability, admitting that the jokey sleeve of the resulting Spiritualized album, Sweet Heart Sweet Light, reflects his state of mind: a giant, bewildered "Huh?"

"The further I get away from the treatment the more I feel it wasn't me making that record," he considers, removing his trademark shades and ceasing initial interview fidgets. "It was like I wasn't in my own head." Although in he debuted the songs to a rapturous reception at the Royal Albert Hall in late 2011(towards the end of treatment, against his better judgment), he hasn't listened to it since, and seems genuinely surprised to be told he's made a great record, perhaps the best since Ladies and Gentlemen. "It was made in such weird conditions, it's hard for me to get a handle on it."

The idea was to make a pop album, to make it easier on himself, but it turned out to be the hardest thing he'd ever done. Just making the record was part of the treatment. "I needed to see an end point." He has emerged with a distillation of Spiritualized and Spacemen 3, but also the finer elements of rock'n'roll, the music he holds dear, everything from gospel to Can to Leonard Cohen to the Velvet Underground and the Beach Boys, even, on the spectral Freedom, middle-of-the-road pop balladry.

"I wanted to touch on all those areas of music, but I didn't want to leave rock'n'roll," he says. "I told myself I was going to embrace melody and harmony, and that in itself wasn't comfortable. But I didn't want to be in comfortable place. I listened to a lot of records that were just collections of songs, like Iggy Pop and James Williamson's Kill City, the 1970s Link Wray albums and the Modern Lovers. Something where you can play it in the car, where you don't need a great understanding of music to know what's going on."

Although familiar themes remain – religious imagery (which isn't down to a belief in God, but the simple language of rock'n'roll, "Heaven sent me an angel" etcetera) and narcotic imagery (although notably less than usual), there's a new tenderness in songs such as Hey Jane, which refers to the "love of my life", and the lovely Little Girl, about the preciousness of childhood.

"It felt like a sentimental record," he admits. "The reason I've got my daughter on this record [on Life Is a Problem] isn't to launch her career! It's the passage of time. I like the bit where she chuckles. We had the window open, and you can hear the birds singing and her voice. It was that idea that things inevitably get pushed back into time, but it doesn't stop you loving them and wanting to hold on to them."

This isn't just a recovering artist getting dewy about things around him, although Pierce admits that his health, age and a sense of mortality have had an impact. His worry is that the essence of rock'n'roll will one day be forgotten. He explains that if we went back to Woodstock now, we'd relate to the people there, but if we went back to the Great Gatsby era of the 1920s, it would be a foreign landscape.

"This amazing thing is slowly slipping further and further away," he sighs, "and I wanted to tell everyone about that." For all the druggy imagery, Pierce's big addiction is music, and he is passionate, insightful and opinionated about what differentiates good from bad, and what he is forever trying – even if the process risks his health – to capture.

"The difference between Patsy Cline, who can melt your heart and absolutely bring you to tears, and someone doing Patsy Cline kinda stuff in a bar is radical," he explains. "One's completely ignorable and the other's Patsy Cline, and the difference might be the timbre of the voice. I mean, now we can download the whole Neu! or Steve Reich catalogue immediately. People can have their lives stuffed with music, but that's not the same as it knocking you sideways and becoming part of who you are. And that's what I was becoming melancholic about, not music disappearing, but that magical, mystical thing."

Pierce grew up up in Rugby in a single-parent family and the trajectory of his life was altered by Iggy and the Stooges' Raw Power, the first record he bought. "It was the silver pants," he chuckles, remembering how he came across it in Boots the Chemist. "I got lucky," he grins. "It seemed to fall from the sky."

Spacemen 3 was an attempt to combine Iggy with pal Pete Kember's favourite band the Cramps, but through "ineptitude" they invented something else: hugely influential space/drone-rock. In Rugby, the Spacemen offered an alternative to the "straight option in life of going into town chasing girls and whatever". When they went national, they united thousands of similarly alienated youth under the irresistible counter-cultural slogan, "For All the Fucked Up Children Of This World, We Give You Spacemen 3".

Although Pierce and Kember fell out, he sees Spiritualized as a gradual evolution of this life's work, and is bemused by criticisms of his output (seven albums in the last 20 years), and tendency to polish and hone his sound (adding gospel choirs or flugel horns) with every album.

"The word recently was 'tinkering'," he grumbles, "like I'm just pushing things around and don't know what I'm doing. Because they're my own songs I feel I've got to get it right. I still can't play guitar well, but I try to play it like it's the most important thing in my life. It's about making what satisfies me in music, and eventually you get to a point where you throw things in the air and they land in the same place."

Pierce's quest is to avoid the "headlong rush towards it being all over" by making new classic music "or at least pushing the vehicle in the right direction". While treasuring the past to edge forward, he is scathing of the boom industry of "middle-aged men trying to pretend that they're kids" and nostalgic "homing in on big moments" in pop history, which is devaluing rock'n'roll.

Pierce refuses offers to reform Spacemen 3, and his attitude hardened in 2009, when Spiritualized played five acclaimed performances of Ladies And Gentlemen. "They were glorious, absolutely ecstatic shows," he admits. "There's no way we could have done shows like that when we did the album. But there was a little voice in my head saying 'Don't get involved in this', because it felt like being in the catering industry. I can categorically say we lost money doing those shows. We had 50 musicians on stage! I'm not looking to be uncomfortable, but doing things for the money is the death of everything. Music is the most important thing."

And so, one of the last great outsiders ploughs his singular furrow, off into the rock'n'roll cosmos, now without so much as a drink. Not that this situation may remain too long. Asked what he fancies once the doctors give him the OK, he nods towards a shelf of wine behind us and chuckles mischievously. "That nice little stack right there."

Sweet Heart, Sweet Light is released on Double Six/Domino on 16 April. Spiritualized play Wilderness Festival, Oxfordshire, 10-12 August and Bestival, Isle Of Wight, 6-9 September

• This article was corrected on 13 April 2012 because the original called Spiritualized's 2008 album Songs From A&E.