After once being relegated to the shelves of obscure health markets, probiotics are officially everywhere. They’ve popped up online and in grocery and drug stores — and people are buying them in greater and greater numbers. According to a 2012 study by the National Center for Complementary and Integrative Health, 3.9 million people have consumed probiotics and/or prebiotics in supplement form. That’s four times as many people taking the supplement than in 2007, just five…