Can I use medical marijuana in another state?

Probably not.  A Pennsylvania medical marijuana card allows a person to purchase and use marijuana in Pennsylvania, but Pennsylvania cannot give a person permission to use marijuana in another state.  It must be emphasized that marijuana remains illegal under Federal law.  A person could be prosecuted at any point for marijuana possession.  At this time, the Federal government has chosen to not prosecute marijuana cases as long as the person is following state marijuana laws.  The Federal government appears to only be prosecuting large marijuana growing operations that intend to sell the marijuana illegally.

While the Federal government may not be prosecuting the use of medical marijuana, many states do not accept medical marijuana and do prosecute people for marijuana possession.  For example, an Arizona medical marijuana user was traveling through Iowa when she was stopped for a minor traffic violation.  The woman, thinking that she was following the law, immediately told the officers that there was marijuana in her car and that she had a medical marijuana card that allowed her to lawfully possess marijuana.  Iowa did not accept the Arizona license, so the woman was prosecuted for marijuana possession.  The woman was then convicted at a trial and sentenced.

A Pennsylvania medical marijuana card only allows a person to buy and use marijuana within Pennsylvania.  Traveling with marijuana outside of Pennsylvania can result in criminal charges being filed.