The spelling can vary but it is frequently spelled marihuana in law. The laws usually provide that "marihuana" is interchangeable with "marijuana."

Example:

"The Michigan Medical Marihuana Program (MMMP) is a state registry program within the Health Professions Licensing Division in the Bureau of Health Care Services at the Michigan Department of Licensing and Regulatory Affairs. The program administers the Michigan Medical Marihuana Act as approved by Michigan voters on November 4, 2008"

Excerpts

Section 333.26424
MICHIGAN MEDICAL MARIHUANA ACT
Initiated Law 1 of 2008

4. Protections for the Medical Use of Marihuana.

Sec. 4. (a) A qualifying patient who has been issued and possesses a registry identification card shall not be subject to arrest, prosecution, or penalty in any manner, or denied any right or privilege, including but not limited to civil penalty or disciplinary action by a business or occupational or professional licensing board or bureau, for the medical use of marihuana in accordance with this act, provided that the qualifying patient possesses an amount of marihuana that does not exceed 2.5 ounces of usable marihuana, and, if the...

(1) 2.5 ounces of usable marihuana for each qualifying patient to whom he or she is connected through the department's registration process; and...