Normal dosage is 20mg to 40mg for Celexa. The possibility exists for having withdrawal symptoms (HA, nausea, diarrhea) if it is suddenly stopped. Lexapro is the 'improved' version of Celexa (minus the ineffective molecules).

It is unlikely that a person could overdose on 15 of 0.25mg Xanax. However depending on what else they took it with (alcohol, other sedatives, narcotics) and their medical problems (COPD, liver disease,etc.) it might be possible. The average healthy adult would probably just be really sleepy for a while. I've seen people take incredible amounts of benzos with no ill effects once they woke up in the ICU. Obviously I don't suggest trying it. For a normal adult a lethal overdosage of Xanax would be pretty hard to do by accident.