Leonard Nimoy was an alcoholic. It started back when he played Spock and continued for two decades. Then, after he stopped drinking, he somehow wrote an entire autobiography without mentioning his 20 years of being a drunk. Then, five years after the autobiography, he decided to reveal his secret on a Star Trek promotional video, Mind Meld. 

In the end, the fullest print account I’ve come across of Leonard Nimoy’s drunk period is in William Shatner’s autobiography (Up Till Now, 2008), where a few pages are handed over to Nimoy for a long quote getting the episode down for the record.

The Nimoy drunk period: 1968 – 1989.
(Nimoy names the end date, but the start date is an estimate based on this quote: “I started drinking regularly, ritually, during the second or third year of our series.” The third season began shooting in spring 1968.)