Does Mirtazapine Cause Weight Gain In A Diabetic?

I'm currently taking Mirtazapine to help as a sleep aid and general relaxer as I'm a hyper personality. The doctor has prescribed 1/2 tablet in the morning and one tablet at night. The problem is, my insurance will not permit more than 30, 30mg tablets a prescription and I need 45. Reading about alternatives, i see its for depression and anorexia which I am neither. And being a diabetic, I don't need the weight gains to go along with taking this drug.
What should I do?