PolitiFact rates this claim as mostly false. I rate it as just another lie, one that fits a pattern. Time and time again we’ve seen Trump promise something, do nothing to make it happen and just declare that he has fulfilled his promise anyway. He knows his followers don’t really care whether it’s true or not. That’s not why they support him. They support him because he hates the same people they do and gives them permission to do so openly.
Source: Trump Lies About Drug Prices Going Down | Ed Brayton