Carlos Santana Recalls Being 'Higher Than an Astronaut's Butt' at Woodstock Thanks to Jerry Garcia: 'It Was a Real Test' (Exclusive)

Carlos Santana's set at Woodstock helped make him a star — but it almost didn't happen.

The legendary guitarist, 75, opens up to PEOPLE in this week's issue about the fateful festival, and the ways in which he had to battle through a substance-induced haze to make it through.

Santana and his band, which to this day bears his last name, were still up-and-comers in August 1969, when promoter Bill Graham helped get them on the Woodstock bill before they’d even released their debut album.

The rockers were slated to play the second day of the festival — but right before they took the stage, Grateful Dead frontman Jerry Garcia offered up some hallucinogens, which Santana says was probably peyote, mescaline, ayahuasca or mushrooms.

"It was a real test, and the test was, 'You are higher than an astronaut's butt right now with whatever Jerry Garcia shared with me,'" Santana recalls. "When I came out onstage, I was like, 'Oh my God, am I going to be able to play? I can't even touch my nose.'"

The "Smooth" guitarist — who shares his life story in the upcoming Sony Pictures Classics documentary Carlos, premiering June 17 at the Tribeca Film Festival— says that as soon he took Garcia’s "medicine," he started seeing colors swirling all around him, and had to look to a higher power to make it through.

"It was like being inside a kaleidoscope," he says. "And then somebody told me, 'Trust in God. Just ask him to keep you in time and in tune.' So I said, 'God, I really believe in you. If you help me right now, I won't poo my pants in front of everybody.’ Next thing I knew, we hit the notes and the people went, 'Wooo!'"

Santana says he remembers Sly Stone's set being the "absolute best," and Jimi Hendrix coming in behind him as a close second. His band, though, was third.

“Anybody else had to fight for number three with me and with our band," he says. "When I look at the video, and even though I was there, it was Sly Stone, Jimi Hendrix and Santana, and then there was everybody else."

The band's eight-song set, though brief, was impactful, and soon, they'd skyrocketed to international success.

Santana says he was "absolutely not prepared" for the fame that followed, and was even warned by Graham about what was to come from life as a rock star.

"Bill Graham told me, 'After Woodstock your head is going to be so big you’re going to need a shoehorn to walk into a room.' And we were like, 'Man, we’re not like that.' And he goes, 'Trust me,'" Santana recalls. "Next thing I knew, we received our first royalty check, and I was like, 'Oh my God, look at all the zeros to the right.' I said, 'Take the check to Mom to get the house like you promised you were going to do when you were 7 years old in Tijuana. Go get the house, get the refrigerator, get everything you need.'"

For more on Carlos Santana, pick up the latest issue of PEOPLE, on newsstands everywhere Friday.