What is the ruling on purchasing nutritional supplements from the United States, about which there is doubt regarding their permissibility or their containing pork fat or alcohol, until their suitability for Muslims is verified, and then consuming them?

It is permissible to buy and sell food products made by the People of the Book because the basic principle is that their food is lawful, unless there is a strong presumption that it contains unlawful ingredients. If such a strong presumption exists, then the original principle conflicts with the strong presumption, which is a matter of scholarly ijtihad (independent reasoning). If the use of unlawful ingredients is widespread in the lands of the People of the Book, then one must exercise caution and refrain from purchasing until assured of its lawfulness.