Matt Damon has/had long term health implications from one of his weight shift stints for a role. Albeit this was done without professional guidance. 

Even with professional guidance rapid weight loss, and possibly gain, can have negative health effects on metabolism and hormones. See: [Harvard Health Publishing - Lessons from “The Biggest Loser”](https://www.health.harvard.edu/diet-and-weight-loss/lessons-from-the-biggest-loser). 

&gt;&gt; Damon was told by doctors that he was fortunate his heart did not shrink. The actor was on medication for several years to correct the stress inflicted on his adrenal gland, but he maintained it was worthwhile to properly reflect his character's anguish and to show the industry how committed he was to the role.