When I first began following the cannabis industry in early 2013, every single stock traded exclusively over-the-counter ("OTC"). Many of them didn't even file with the SEC! Fortunately, the market has evolved, and today most cannabis stocks trade on junior and senior exchanges in Canada as well as major exchanges in the U.S.
Those wishing to invest in companies that grow, process or sell cannabis but that are unwilling to invest in Canada or on the OTC face a very narrow universe, as major exchanges currently don't permit direct federally illegal cannabis companies to list.