If a doctor is licensed in Puerto Rico can he practice in the us?

They are practicing in the US while in Puerto Rico. If they can get a license in one of the states, they can practice there. It's the same for a doctor of any state practicing in another state. 

Medical licenses are issued at the state, not the national level.  Some states have reciprocity with others, some don't.  Puerto Rico is a US territory, so it is part of the US, despite trump supporters' failure to grasp this concept.  So a state may have reciprocity with PR, or it may require additional testing in order to get a license. 

Medical professional licenses in the USA are issued by state or Territory so they would have to apply for a license to practice medicine in the one to which he wants to move. 

"Even doctors who have been practicing in Puerto Rico for years and years may have to take additional examinations if they want to practice in the majority of states in the US."

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