I don't know of any specific reason why you would have to stop taking mirtazapine while talking Lithium. But this is something your doctor should have addressed when prescribing the Lithium for you. If the same doctor prescribes both drugs, then he or she ought to have said something if you were to stop the mirtazapine. If two different doctors prescribe the two meds, each one needs to know all the prescription drugs you're taking.
For a quick answer based on all your medications, talk to your pharmacist or call the doctor who prescribed Lithium. It's possible your mirtazapine dose might have to be adjusted now that you're on Lithium.
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