I have never had a problem in the United States. That being said, I still would advise againist carrying anything in plastic baggies, especially white powder. With todays climate it not only is a red flag for drugs, but, you have to remember that "white powder" covers anthrax,and certain other chemicals not to be discussed on open forum..Usually in the States, a simple explanation is good enough for Customs, but, I still like carrying them in their original bottles. One word of caution: if you take several different prescriptions, dont put them all in the same bottle, keep them separate.
On international travel from my experience:
Never carry white powder in baggies in Central America, South America, or the Caribbean. too much that could go wrong ..fast. I once had a pound of protein powder seized in ST.Lucia, in the original container...after being surrounded by ill-trained personnel.
Middle East: hit or miss. had the problem with Dubai, never had a problem in Kuwait, Jordan, Iraq, Bahrain....take NOTHING into Saudia Arabia you dont need to survive...period
Europe: been in many Western European countries, only had a problem in France one time, but, no big deal. Dont have a lot of experience in Southern Europe...no problem in Italy.
Africa: only two countries there, so dont know much. hope that helps.