Sherlock surely took the right pill. The cab driver was meming to the one in front of Sherlock with all his body language since the beginning insiting him to take it. he also took it out of his pocket before the other to make his victims give it more importance. add to that he pushs it in front, while the other sat quietly and descretly in the back since the beginning. All these mind/psychological games added together create strong attachement to the wrong one and make it the default choice. This is why the man survived 4 times. Besides that, Holmes wouldn't let the man die without getting the answer or without having the pills analysed to decrypt the criminal mind better. and he got it when the man, out of shame did not answer which is the one. so quickly and intellegently Holmes shifted the question to know about Moriarty.