Might I overdose from promethazine and citalopram if I take it every day?

Unlikely. Not if you take the dose you are prescribed and it is a proper dose (celexa (citalopram) up to 40mg and Promethazine up to 100mg/day) you can get side effects such as sedation. If you are so sedated that you don't remember what you are taking, I guess you could take an extra dose which shouldn't hurt you but if you have a bad reaction, see your doctor immediately or go to the er.