I have a bottle of expired mirtazapine tablets, they expired in 11/16/21. i was wondering if i could still take one as i used to take them for sleep?

I cannot recommend taking a medicine that has been expired for that long. Your question also leads me to think you have not taken this medicine for a long time. You do not say what dose you have so the dose may not be correct for a starting dose (essentially you are starting over with this med). You need to call your doctor and get a new prescription, if your doctor agrees with you re-starting med