From my understanding, pharmaceuticals (8) are almost the same as drugs (5), cross referenced are two. I believe merging these two does no harm, and I would suggest to keep drugs as a synonym and use the more appropriate pharmaceuticals. I am uncertain how to treat drugs that are usually considered illegal substances, but I think If these are within the scope of this website, they can be treated as pharmaceuticals. An example for this kind of question is What chemical properties make LSD so psychoactive?
The other part I am more uncertain about. I understand, that pharmacology (4) (and toxicology) are closely related to medicinal-chemistry (17). I believe, that even in the long run, the scope for pharmacology is too close to medicinal-chemistry and that most of these questions will be tagged with both.
In line with organic, inorganic, physical, etc. -chemistry, I suggest keeping medicinal-chemistry and having pharmacology as a synonym. (In this I think it would be appropriate to create toxicology as a synonym too.)
Looking at the answers given so far, I see that there is consensus about the pharmaceuticals into drugs merger. (I guess we can do that now.)
About the second part there is the question still open, if pharmacology should be merged or deleted.
I personally would keep the tag as a synonym, to avoid having this discussion somewhen in the future again. I do however completely support Greg E.'s answer. I just take this stand out of convenience.