Tyga -- Pens Signature on Joint Full of Weed

Stoners may frown on smoking a joint covered in ink ... but one Tyga fan says it's possible he may break that rule -- after the rapper jotted his John Hancock on the guys weed-filled zig zag.

Tyga was celebrating the opening of his new store Last Kings Saturday in Los Angeles ... and while snapping pics with fans, one guy asked Tyga to sign his ganja stick.

Afterward ... the guy doesn't quite say he'd never smoke it.

We called doctors across the country attempting to get an expert opinion on what kinda damage smoking ink can have on your lungs.