Once you become a registered medical marijuana patient in Pennsylvania it is now illegal under federal law to own a firearm. While licensed use of medical marijuana is legal in PA, it is still illegal under federal law as marijuana is listed as a Schedule I substance.
This means that according to the U.S. Department of Justice Bureau of Alcohol, Tobacco, Firearms and Explosives (BATFE) any medical marijuana patient is considered "unlawful user of or addicted to any controlled substance.” who is prohibited by federal law from the purchase or acquisition, possession, or control of a firearm, according to 18 U.S.C. § 922(g)(3) and 27 C.F.R. § 478.32(a)(3).
So, possession of a medical marijuana card infers for federal purposes that you are an unlawful user of a controlled substance, which disqualifies you from lawfully owning a firearm under federal law. This letter from BATFE dated September 21, 2011 sums up the federal policy regarding medical marijuana and firearms.
So, while this may appear as a strict "no" this matter does still reside in a legal gray area because of the conflict between state and federal law. The PA Department of Health is required by law to protect patient privacy by not posting patient names anywhere, including to the database used to conduct background checks by law enforcement agencies. This means that there is no way to verifythat you are in possession of a medical marijuana card when applying for a firearm or a concealed carry license . You will however be asked when you apply for a firearm if you “use medical marijuana” to which answering “yes” would deny you the license and falsely answering “no” could result in future legal problems from purchasing a firearm with falsifying information.