For how long paroxitine can be used. what are its side effects if used for a long time.?

No data: There is no data about a maximum limit of time. Limitations in particular cases include weight gain, somnolence, intolerance to higher doses with tolerance (as in lost efficacy) in current or high doses. Withdrawals are common when stopped abruptly (and this phenomenon make some patients want to be weaned off). If there is a side effect that keeps you concerned, you should discuss it with your dr.

Depending on episode: If you have one episode of depression you use it for 6 to 12 months if you had two episodes you use it for 4 to 6 years if you had three or more episodes you use it for life unless you develop a severe side effect like rapid heart beat or you have poor liver or kidney functioning. No specific long term side effects were reported of significance.