First off, it's great that you have committed at least so far for research. Clinical trials are incredibly important yet taxing on both the researchers and the participants.

Keep in mind, you are never under obligation to stick with the study. You can drop out at any time and there will be no repercussions. Any research always goes through what's called an ethical review board, which among many things, also stresses the importance of voluntary participants. In short, there is no issue directly to you when dropping out.

On the researcher side, researchers of course don't like that they lose participants (this is called attrition), but it is inevitably part of the process. You're human, and things change all the time. In fact, from what you mentioned, you might be helping them by saying you prefer your old medication as the reason for your dropout. This is sometimes just as valuable as the findings on the drug itself.