Oklahoma has voted to legalise medical marijuana, making it the 30th state to do so.

Individuals 18 and older will be able to obtain a medical marijuana license with a board-certified physician's signature. Minors also can get a license with the approval of a parent or legal guardian signatures from two doctors.

The measure does not tie medical marijuana to any qualifying conditions, unlike many other states that have passed the measure. This will simplify the process to obtain cannabis for medicinal uses.

State Question 788, the legalization of medical marijuana, passed with 57 per cent of the vote, while 43 per cent opposed it.

"I respect the will of the voters in any question placed before them to determine the direction of our state," Oklahoma Governor Mary Fallin said in a statement. "It is our responsibility as state leaders to look out for the health and safety of Oklahoma citizens."

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Governor Fallin said she stood by her stance that "this new law is written so loosely that it opens the door for basically recreational marijuana." House Majority Leader Jon Echols said applications to acquire permits could be out as soon as the end of August.

As well as the now-30 states where medical marijuana is legal, nine states and Washington, DC allow for recreational sales