Do the side effects of remeron (mirtazapine) ever go away if you've switched to another drug?

Yes. Yes, the side effects (sedation, etc) of Remeron (mirtazapine) do go away once you stop the medicine and gone to another drug. But the elimination half-life of Remeron (mirtazapine) is 20-40 hours -- quite long. So depending on your personal metabolism, it could take a week or so before your body has processed it all.