CLEVELAND (Reuters) - Ohio voters will decide in November whether to legalize marijuana use for treating illnesses and for getting high.

Secretary of State Jon Husted said on Wednesday he had certified the group ResponsibleOhio had gathered enough valid signatures to place a constitutional amendment before voters in November.

The proposed amendment's language still needs approval by the Ohio Ballot Board, he said.

The measure would establish a Marijuana Control Commission charged with regulating the growth, sale and taxation of marijuana, similar to legalization plans approved by voters in four other U.S. states.

In November, Oregon and Alaska approved the use of marijuana for recreational purposes in state-regulated systems that will usher in retail pot shops similar to those already operating in Washington state and Colorado.

The District of Columbia has also voted to allow recreational-use marijuana but not retail shops.

About half of U.S. states allow marijuana for medical use.

The vote in Ohio, a heartland state known as a political bellwether in national elections, reflects the shifting landscape for a substance that remains illegal under federal law. Pro-marijuana groups hope to put similar measures before voters in six other U.S. states in 2016.

The Ohio amendment would designate 10 locations for the growing of marijuana and make it legal for people 21 or older to use and possess up to one ounce (28 grams) of the drug and grow up to four marijuana plants for personal use.

ResponsibleOhio handed in 305,591 signatures from half of the state's 88 counties.

The wording of the amendment has drawn criticism from some marijuana legalization supporters who say it creates a marijuana monopoly in the state.

Another ballot measure, an anti-monopoly proposal backed by state lawmakers in June, would effectively block the marijuana measure if passed in November because a citizen-initiated amendment takes 30 days longer to take effect than a legislative one.

(Reporting by Kim Palmer in Cleveland, Ohio; Editing by Eric M. Johnson and Mohammad Zargham)