Over a person's lifetime they are likely to be prescribed more than 14,000 pills. These drugs perform minor miracles day after day, but how much is really known about them? Drug discovery often owes as much to serendipity as to science, and that means much is learnt about how medicines work, or what they do, when they're taken. By investigating the most popular pills people pop, Horizon asks, can they be trusted to do what they are supposed to?