Preventative Medications?

Hi! I have had two chickens die which I could not explain. No one looks ills; everyone is eating, normal stool, etc. We have tried letting them out because our winter has been so mild but still giving them access to their house that is insulted. So I am not sure if that is causing a problem.

Wondering if there is a medication or a preventative vitamin that I could put in their food or water as a precautionary method to help with something they may be lacking.

thanks!