Marijuana is one of the fastest-growing industries in the US. It's also illegal. Federal prohibition has given rise to a number of dedicated funds that are looking to capitalize on the "short window" of opportunity before institutional investors start writing checks. Investors who are patient and willing to take on the risk stand to make huge gains. Legal marijuana is one of the fastest-growing industries in the US. It generates hundreds of millions in tax revenue in states where selling the drug is legal, and it's expected to become a $32 billion global market in the next four years.