Miley Cyrus taught her teen audience in London a very important lesson Tuesday night -- 1) quit smoking cigarettes ... and start smoking weed instead ... because weed will keep you young and get you laid.
Cyrus was performing at the O2 arena when she smelled cigarette smoke and went off on a bizarre rant about how she no longer smokes cigarettes because they're bad for her health and her looks ... but WEED is a whole other story ...
You gotta take a listen -- she also says, "I'm cracked out on B vitamins."
She can't stop. She won't stop.