Zoloft and Effexor are NOT the same. Here is the scoop:
Effexor is an SNRI (Serotonin and Norepinephrine Reuptake Inhibitor). It works on TWO brain neurotransmitters (serotonin and norepinephrine). At lower doses (37.5mg-75mg) it works ONLY on serotonin. Only at higher doses (150mg-300mg) does its "dual mechanism" effect occur. Effexor has a relatively short half-life, meaning that it needs to be taken with greater frequency to keep a plateau amount of medication in one's body. That is why the drug company created Effexor XR (extended-release formula). Effexor can lower the seizure threshold in people who have a history of seizures, but more frequently can cause complications in people who have hypertension (high blood pressure). Effexor is one NASTY medication to quit cold turkey - although it is not dangerous, it is highly uncomfortable. It's one of the worst for abrupt discontinuation, Paxil being notoriously the worst.
Zoloft is an SSRI (Selective Serotonin Reuptake Inhibitor). It works on ONE brain neurotransmitter (serotonin) at all dosage forms (25mg-200mg). It is taken only once a day (normally in the morning) and does not have an extended release formula. The generic is cheap. It has few side effects and people with multiple medical problems can take it. It should not be stopped abruptly; although stopping it abruptly is not dangerous, it is fairly uncomfortable - resulting in numerous physiological and psychological symptoms. For people with high levels of anxiety, the higher doses of Effexor can do more harm than good, as the inhibition of norepinephrine reuptake is thought to play a role in anxiety.
Hope that helps.
I'm a mental health clinician.