Been on 20mg celexa (citalopram) for 5 months. doc.cut me down to 10mg for 2 weeks now due to side effects. is 10mg ib my system now or would still be 20mg?

Likely at10 mg level: Celexa (citalopram) has an elimination half life of ~35 hrs with most metabolized by the liver and ~15% by the kidney. If your liver and kidney are normal and there are no other medicines or foods having inhibitory effects on citalopram metabolism, then after two weeks on 10 mg dose the blood level most likely reflects that dose despite having reached a steady state level on 20 mg for 5 months.

Citalopram: The elimination half-life of citalopram varies from 23-45 hours. At its lengthiest then, this means around 9 days to stabilize at the new level. So by 2 weeks, your system should be at your new 10mg dosing levels.