I just read about Judy Garland on Wikipedia:

"To keep up with the frantic pace of making one film after another, Garland, Rooney, and other young performers were constantly given amphetamines as well as barbiturates to take before going to bed. For Garland, this regular dose of drugs led to addiction and a lifelong struggle and contributed to her eventual demise."

Thanks a lot, Metro-Goldwyn-Mayer!