I am looking for a number of slow accumulative poisons say over a 6 month period that is detectable in autopsy and would slowly but surely kill the patient. The poison would be hopefully something a doctor or nurse could obtain as the poisoner will be a medic. Thanks!
Low levels of cyanide. It's not lethal in small enough doses, but will build up if the dose is large enough.
Can easily be obtained from natural sources such as cassava root (the root that tapioca is made out of). So basically improperly processed cassava root will be your friend (you may need to sweeten it because cyanide tastes bitter).
If your patient could spend a lot of time around artwork, consider some classic oil paints. Your antagonist, if suspected at all, could use the excuse, "I didn't know those were deadly!"
I remember seeing an old movie where someone was killed over time by, I think it was called, Paris red paint in his home. But having viewed the link above, I wonder if it was Paris green.
Chronic lack of B12? I suspect you are thinking about niacin, lack of which is pellagra, takes forever, and is easily reversible by eating meat.
In this hypothetical I would go with polonium or ethylene glycol because you're looking for something that CAN be detected. Warfarin and nicotine also come to mind. Have you tried watching TV?