Some people on chemotherapy have different issues, i.e. a metallic taste or different smells.
It can be hormonal. When I was taking The Pill I couldn't smell anything other than garbage
although I didn't realise until I had stopped - after a few weeks I could smell the roses
Also, I think that I smell whiffy but DH tells me that he can't smell anything different
- see 'Oh Errrr' thread ;-).