Gene Tunney's daughter murdered her husband in 1970, when the former champion was 72. It is often cited that incident that tipped him over the edge, and into alcoholism. https://www.nytimes.com/1970/03/30/...-of-slaying-husband-is-held-by-police-in.html However you hear the occasional rumor of his heavy drinking going as far back as World War II.
Larry Holmes liked to drink beer even when he was active. Before the Spinks fight he was drinking beer ringside at some HW fight , I can't remember which one.
Use the internet. I believe he defeated Eleider Alvarez after breaking a woman's nose and had some weird incident on a plane a couple of years ago.
Joe Frazier drank a lot throughout his career and was a big time boozer by the time of the 2nd Foreman fight. Allegedly drunk the night before from what I've read. He got a few DUI's I believe also and hit the bar scene a lot during his singing career after the FOTC I think. All hazy but I do know he loved to drink up and it's not mentioned enough that Frazier losing to Ali and Foreman was as much alcohol and poor habits as it was skill. Everybody get's an alibi but Frazier is assumed to just have lost because he wasn't good enough when the facts don't reflect this entirely. Joe couldn't turn in a performance like the FOTC not because "He left it all out there" I reckon he just mistreated himself from the late 60s onwards and aged up bad. Joe was a bad man but it's got to just be you and the other guy in there and when you got the devil on your shoulder....
I know that Jermain Taylor has struggled with alcoholism, which has also resulted in several legal issues.
Machen is a figure i want to know more about, his struggle with mental illness is something I really empathize with.