the medicines in USA contain an ingredient magnesium serrate that could be derived from animal or plant sources, Muslim websites informing about Halaal/Haraam food state that it is Haraam if derived from animal source, if we mail a pharmaceutical industry about its status (animal or plant source) they don't bother to reply, can we use the medicines that come from America and non-Muslim countries. Even if it is animal source we don't know how it was slaughtered and which animal (it is difficult to get information about source of this ingredient).

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

It is permissible for a Muslim to use or consume the medicine which his doctor prescribes for him even if he does not know its ingredients. In principle all things are permitted, and investigating the ingredients of a medicine could cause hardship and difficulty. Allaah Says (what means): {…and has not placed upon you in the religion any difficulty.}[Quran 22:78]. However, if a Muslim knows that this medicine contains what is forbidden like dead meat, pork or any other impure ingredient, then it is not permissible for him to use or consume it. 

Allaah Knows best.