Yes, this is a safe combination, with a caveat. Most individuals are heavily sedated for the initiation of mirtazapine treatment. This effect usually disappears as the patient adjusts to the mirtazapine. If you still find a powerful sedative effect from mirtazapine, you should be advised to keep the dose of codeine low. If you are adjusted to the sedative effect, you can safely take whatever dose of codeine your doctor advised.