Jared Leto lost 28 lbs. to play a heroin addict in the 2000 film Requiem for a Dream.
He gained 67 lbs. to play Mark David Chapman, the man who killed John Lennon, in Chapter 27 (2007).
For Dallas Buyers Club (2013), he once again dropped weight for a role - this time 30 lbs. to play an AIDS-positive transgender woman.
Matt Damon reportedly lost 40 lbs. in just 100 days for his role as a soldier addicted to opiates in Courage Under Fire (1996). The weight loss caused so much stress on his adrenal gland that he took medication to correct it for more than a year after filming.
For The Informant (2009), he packed on 30 lbs. via junk food to play a corporate whistleblower.
He then bulked up with solid muscle for his role in Elysium (2013).