ML,
Paxil and Celexa are both antidepressants from the same class , SSRI's, which primarily increase the availability of serotonin in the brain. Each drug is chemically unique and even though they act in a similar manner in regards to mechanism of action, there are some differences. You are right that if someone fails a class of agents, we usually choose one from another class.
Sometimes a patient may respond to one SSRI and not another. Side effects also tend to be similar within the same class but do have some important differences. A different class of antidepressant should be considered if possible.
Remember that this information is intended for educational purposes only. Please see your personal physician for specific health concerns.
Sincerely,
HFHS MD-JM
Keywords: SSRI, antidepressants