Your doctor or prescribing doctor should have really told you how and when to take the prescribed dose?? Did he/she not?? That is bad.
In the product information, citalopram (celexa) is not really effected by food (i.e. absorption isn't effected) so it really does not matter what time of the day you take it. I am assuming that you have just recently started treatment since you have 10 mg tabs/caps, but are now titrating up to 20 mg a day yes? In the that case, you will probably need to take this AM and PM and spread it out a bit.
I would suggest seeing your doctor again to evaluate how the dose is working for you. If 20 mg is working well and you don't need to go up to 40 mg, then maybe it would be better to be prescribed the 20 mg caps/tabs so you can take it just once in the morning.
Good luck sweety
