Is it okay to put my horse back on hydroxyzine as he is on Metformin?

It is the only thing that helps to control his itchy skin during the summer. Over the years I have tried addressing it through diet (increased flax, spirulina, chondroitin, etc.) and fly gear. 

Any insight appreciated. 
--
Mara
NJ
2020
https://ecir.groups.io/g/CaseHistory/files/Mara%20and%20Enzo/?ord=dirname&d=asc