The most amusing interactions in pharmacies are ironic ones:
Customer: Hi! Can I get some Phenergan 25mg tablets please
Me: Sure, it's a pharmacist-only medicine so I'll just need to ask you a few questions.
Customer: You don't need to do that, I am a homeopath*
Me: Oh that's excellent! Let me just go dilute that for you so it conforms with your pseudo-scientific hocus pocus.
Disclaimer: Not all of the above interaction took place, but that goes without saying.
*Homeopaths believe that by diluting a substance to the point of it being just water, it will enhance the substance's effect. For example, a standard 30C homeopathic dilution is tantamount to putting 1ml (around 12 drops) of active substance in a sphere of water that has a diameter of about 131.1 light years in length.