Mike Reed, a musician and Uber driver in Arizona, said he quit drinking alcohol more than a decade ago when his roommates got so fed up with his unruly behavior that they threatened to kick him out.

Sobriety became such a core part of Mr. Reed’s identity that he launched an online dating website called “Single & Sober,” but in 2020, Mr. Reed, a Navy veteran, said he found himself struggling as his sister, who had Down syndrome, was dying of cancer.

Mr. Reed, 43, began smoking marijuana. More recently, he went to a clinic for infusions of ketamine, and tried tiny doses of psychoactive mushrooms. Mr. Reed said those substances improved his mood — and he still regards himself as sober, because he remains alcohol free.

Notions of what constitutes sobriety and problematic substance use have grown more flexible in recent years as younger Americans have shunned alcohol in increasing numbers while embracing cannabis and psychedelics — a phenomenon that alarms some addiction experts.

Not long ago, sobriety was broadly understood to mean abstaining from all intoxicating substances, and the term was often associated with people who had overcome severe forms of addiction. These days, it is used more expansively, including by people who have quit drinking alcohol but consume what they deem moderate amounts of other substances, including marijuana and mushrooms.

“Just because someone has a drinking problem doesn’t mean they have a problem with every single thing,” Mr. Reed said.