This map shows every state that has legalized marijuana

Skye Gould/Business Insider

• Recreational marijuana is legal in nine states and medical marijuana is legal in 29 states.
• Vermont became the first state to legalize marijuana through the legislature, rather than a ballot initiative, in January 2018.
• A recent Gallup poll showed that 64% of Americans support legalization.

One of the world's biggest cash crops - marijuana - is sweeping the US.

It's legal for adults over the age of 21 to smoke marijuana without a doctor's letter in nine states. Medical marijuana is legal in 29 states. Vermont became the first state to legalize marijuana through the legislature, rather than a ballot initiative when Republican Governor Phil Scott signed the bill into law in January.

Marijuana prohibition began 80 years ago when the federal government put a ban on the sale, cultivation, and use of the cannabis plant. It remains illegal on the federal level.

Despite the efforts of Attorney General Jeff Sessions, who has been on a crusade to stamp out legal marijuana since his appointment, the industry is exploding.

Legal marijuana sales were expected to hit $9.7 billion in North America in 2017, according to a report from Arcview Market Research. Arcview has not yet released final numbers for the year, but its forecast represents a 33% increase over 2016, shattering previous expectations about how quickly the industry could grow in the face of federal prohibition.

The report also predicted that the entire legal marijuana market will reach $24.5 billion in sales - a 28% annual growth rate - by 2021, as more state-legal markets come online.

Support for marijuana legalization reached new highs in 2017. A Gallup poll showed that 64% of Americans favor legalization, and a majority of Republicans back it for the first time.