Debbie Reynolds had just landed the role of a lifetime—and she was exhausted. The 19-year-old had been cast as Kathy Selden, the female lead in Singin’ in the Rain, and she had big shoes to fill. Her partner was none other than the very seasoned, astonishingly talented Gene Kelly, and Reynolds was expected to match him step to step.

Reynolds was up to the challenge, but the grueling rehearsal schedule and pressure soon began destroying her health. When her doctor advised her to take a week off of work, MGM studio chief Arthur Freed told her to go to a different doctor.

In her 2013 memoir, Reynolds recalled how Freed instructed her to get “vitamin shots” from his doctor. “These were possibly the same ‘vitamins’ that ruined Judy Garland,”she wrote.

Reynolds had just discovered one of Old Hollywood’s dirty little secrets—that drugs fueled its classic films. Between the 1920s and 1960s, Hollywood studios created some of history’s greatest films. But they often did so at the cost of their stars’ health.

Despite the pressure, Reynolds stuck with her own physician. “My doctor insisted that I stay in bed,” she wrote. “That decision may have saved me from a life on stimulants.”

There was no official policy of drug use within Hollywood studios, but the carefully regimented system that cultivated movie stars often relied on behind-the-scenes drug use to power actors through unthinkably long days.

Child actors were supposed to be subject to strict labor laws that regulated the hours they spent on set; however, actors like Elizabeth Taylor and Shirley Temple recalled that directors and studio heads always tried to push the boundaries of those hours. Seeing kids leave a set early surprised Taylor later in life: “We didn’t have that at MGM,”she said. In herautobiography, Temple recalls the entire studio celebrating her 18th birthday—by working her all night long.

Though Taylor and Temple both got through their child stardom without drugs, Judy Garland did not. She was introduced to “pep pills” by hermother, who insisted The Wizard of Oz actor take them in order to give an energetic performance. Over the years, as Garland became a bigger star, she was prescribed pills by MGM studio doctors to control both her weight and her energy levels.

“They’d give [me and Mickey Rooney] pills to keep us on our feet long after we were exhausted,” Garlandtold biographer Paul Donnelley. “Then they’d take us to the studio hospital and knock us out with sleeping pills…then after four hours they’d wake us up and give us the pep pills again so we could work 72 hours in a row. Half of the time we were hanging from the ceiling but it was a way of life for us.”

For Garland, who found it difficult to deal with the pressure of being one of MGM’s most visible and hardest-working stars, this regimen led to full-blown addiction and a series of nervous breakdowns. Shedied of a drug overdose at age 47.

Other stars, like actress Joanna Moore, wereprescribed amphetamines or “vitamin shots” to control their weight. And for many women caught in the star system, which demanded physical perfection and performance, taking pills prescribed by studio doctors didn’t feel optional.

“In those days,” Twentieth Century Fox doctor Lee Siegeltold Marilyn Monroe biographer Anthony Summers, “pills were seen as another tool to keep stars working. The doctors were caught in the middle. If one doctor would not prescribe, there was always another who would….everyone was using pills.”

Actors weren’t the only ones taking drugs in Hollywood. Legendary director and producer David O. Selznick notoriously depended on a steady diet of Benzedrine (an amphetamine) to get him through the long hours of making movies like Gone With the Wind. Evelyn Keyes, an actor on set,recalls Selznick “crushing up benzedrine and licking the pieces from the palm of his hand, a grain at a time,” on set. Director Carol Reed and much of his crew reportedly took large amounts of amphetamines to keep up with the quick-paced production schedule of another classic film, The Third Man.

Other stars struggled with drug addiction, too, but got their drugs from outside the studio system. And Hollywood wasn’t the only industry with an addiction to pills. Amphetamines actually increased in popularity after World War II due to its widespread use (and abuse) in the military. And by the 1960s, it was a full-fledged epidemic, with so-called “rainbow diet pills” (actually a potent cocktails of sedatives and stimulants) commonly prescribed by doctors.

In 1970, amphetamine use was dramatically curtailed by the Controlled Substances Act, whichacknowledged its addictive properties. By then, Hollywood had moved on to other stimulants, likecocaine, and the studio system that often provided drugs to actors for the sake of a good performance had lost much of its power.

But you need only look to the silver screen for the legacy of drug abuse in Hollywood’s classic films—it’s right there in the energetic work of some of the most glittering directors and stars of the era.