Potlatch2 does have amenity=pharmacy (http://wiki.openstreetmap.org/wiki/Tag:amenity%3Dpharmacy), which in general will correspond to places where you can get medicines prescribed by your doctor from a person with a specific qualification for this purpose. My understanding is that amenity=chemist is generally used for shops selling similar products (toothpaste, asprin, cold remedies), but not supervised by a qualified pharmacist, nor able to dispense regulated drugs and medicines.

Thus in Britain we have a chain of stores called Boots (it used to be called Boots the Chemist). Some of its shops are pharmacies, others are chemists, whilst the larger ones will usually include a pharmacy (and an optician and even perhaps a dentist). Another chain, Superdrug, used to have no pharmacies at all. In most European countries amenity=pharmacy will be appropriate.