Each country sets its own laws and regulations regarding citizenship, and until 1991 only U.S. citizens could be licensed to practice medicine in the country. Once Congress passed the Immigration Act of 1990, foreign medical professionals were permitted to obtain temporary visas to enter the U.S. to attend medical school and to practice in their respective fields.

Until 1967, dual citizenship was not permitted in the United States. Today, foreign medical professionals who become naturalized citizens can retain their citizenship in their home country (if its laws allow it) without compromising their U.S. citizenship.

But it is possible to lose U.S. citizenship. For individuals born on U.S. soil, their citizenship can never be revoked; they can only renounce it voluntarily by engaging in conduct with the intent of giving up U.S. citizenship. This is called expatriation. Naturalized citizens can also voluntarily renounce their U.S. citizenship, however their citizenship can be legally revoked depending on the circumstances.

According to the U.S. Department of State, it is presumed that a U.S. citizen intends to renounce their citizenship, or, for naturalized individuals, may lose their citizenship, under the following circumstances:

• they formally renounce their U.S. citizenship at a consular office;

• they become part of the government of a foreign country where their position is policy-level;

• they serve in a foreign military which is engaged in hostile action against the United States;

• they are convicted of treason.

To make sure U.S. citizenship is retained, especially if the nature of a stay in or ties to a foreign country could potentially trigger a presumption of expatriation, a U.S. citizen can assert their citizenship status by 1) submitting a written statement declaring intent to retain U.S. citizenship to an American consulate or the Department of State; 2) continuing to pay U.S. taxes; 3) obtaining U.S. passports; 4) maintaining property and other personal ties to the U.S.