I've been on Mirtazapine, Propranolol, & Diazepam, for 6 months, and my GP signs me off for a month at a time, and I have to go back to see him every 3-4 weeks. My GP has indicated that I should stay off work until the end of the year (which is doing my head in - but that's another story).

If you explain to your GP how you feel and that you cannot work whilst on those meds, he should give you a certificate. Ask your GP how long he thinks you will need to stay on medication for, to give you an idea of how long you might be off work for altogether (will also give your employer a better idea of how to cover your absence).