hello there! paroxetine is SSRI selective serotonin reuptake inhibitors and wellbutrin is bupropion, both are antidepressants. now you said one tablet of paroxetine and you stopped. so let me tell you these drugs show their effect after at least using for a week and then they remain in a level enough to relieve the depressive symptoms. bupropion is also an atidepressant. but ssri is a bit safer and renovated drug and a better antidepresant near my experience. so here is the solution. treat the diarrhoea symptomatically as it will be fine one your body gets used to the drug and continue the paroxitines and wait for the effect. it will come eventually. but if you can not tolerated the adverse effects of paroxetine then you can switch to bupropion any time. it is an antindepressant too you wont feel a difference cheers