Huh, when I switched from Cymbalta to Zoloft (sertraline) recently, my p-doc had me start taking Zoloft at 25 mg the first week that I went down to 30 mg of Cymbalta. Then the next week I stayed at 30 mg of Cymbalta and started with 50 mg of Zoloft. Then next week, I went up to 75 mg of Zoloft and stopped the Cymbalta. I was also on 150 mg of Wellbutrin at the time (and still am). I still felt some withdrawal symptoms after I stopped the 30 mg, even though I was on an SSRI and Wellbutrin, which affects neurepinephrine like Cymbalta (and my p-doc said I shouldn't have side effects). For me, I was just really achy, like I had the flu, had headaches, and was very irritiable, like I started screaming at my husband about something, and I never do that. Passed after about a week.

I'd see check with your p-doc to see if you could start the citalopram while still on the Cymbalta. Or at least don't go a week without taking anything--not sure of the point of that because Cymbalta is out of the system quickly (which is why the withdrawal is hard).