CONCORD, N.H. — As New Hampshire again considers whether to legalize medical marijuana, neighboring states offer lessons about the complexities of implementing such a law.

The New Hampshire Legislature passed three medical marijuana bills in previous years, all vetoed by former governor John Lynch. This time, Governor Maggie Hassan’s endorsement could tip the scales. Eighteen states and the District of Columbia allow people who are sick or in chronic pain to legally buy and use medical marijuana, but laws vary widely.