I have yet to see a reputable pharmacy here in the US do this kind of thing--maybe they do in other countries. In fact, most "compounding pharmacies" (i.e. pharmacies that regularly do such custom-blending) that I know about do not even advertise the fact that they in fact do such custom-blending. You typically have to find out from a physician which ones undertake such work.
Then the issue isn't really speech in and of itself; it's dissemination of misleading or outright false information. If it can be proved as such, then the first amendment doesn't apply. But it has nothing to do with whether or not the communication is "commercial". If it propagates false information, and that information can do harm, then it should be evaluated on that basis and only that basis.