I can smell but i cannot taste could low serotonin cause this. i can taste only few things. i am a little depressed some times.

Smell: Its unlikely that low serotonin has anything to do with this. Metformin has been said to cause a metallic taste, and there are a few claims it caused taste loss. But smell and taste are very primitive, central sensations and I would look for a central nervous system issue. You need a good, thorough neurological workup. Make an appointment with a neurologist sooner than later.