The Arizona Court of Appeals ruled that people with medical marijuana prescriptions in another state are free to use their medicinal cannabis while in Arizona. The case revolved around a California man who was arrested for marijuana possession in Arizona, but his cannabis was bought using a prescription he had from his home state. Prosecutors argued that having a prescription from another state did not allow for the same immunity as Arizona residents with a prescription, but the court obviously disagreed. The actual law that legalized medical marijuana in the state had a clause that grants immunity to "visiting qualifying patients."
The court said as long as a person can provide documentation that they are allowed medical marijuana under the home state's law, they're free to use it in Arizona.
However, out-of-state medical marijuana prescriptions do not allow users to purchase medicinal cannabis in Arizona. So you'll still have to buy your stash where you live.
(h/t AZ Central)