PUERTO VALLARTA, Mexico —  The tag on her white lab coat read “professional pharmacist,” and the framed health and safety certificates lining the walls behind her gave the drugstore an air of legitimacy. That pretense faded seconds later, when she was asked for controlled medications — and got on her hands and knees to pop open a hidden panel under the counter. She rooted around for a minute and emerged with two sealed bottles. “These are from licensed laboratories,” she said. “The problem is when you’re buying from a laboratory that’s not certified.” One of those bottles — sold as Adderall — tested positive for methamphetamine.