While there is some work done on matching people up with specific drugs, this is still very much in the research phase.

Still, for many people, it's more trial and error than it needs to be. Many family doctors still don't match up symptoms with neurotransmitters, and then put people on the wrong class of drugs altogether (e.g. something like Wellbutrin which works on norepinephrine and dopamine instead of an SSRI for anxiety which is more associated with serotonin).

Part of the reason is that new drugs keep coming out so it's difficult to keep up, and eventually they all start to blur together.

The best practical predictor of success on a given medication today is if you have a close relative who's had success on that medication.