Can i wean off of effexor (venlafaxine) xr 35mg, and take zoloft 25mg at the same time, so there is no lapse in treatment? and how long until zoloft starts working?

Yes, if doc agrees: always discuss your plan of care with ur doc. it's most common 4 docs 2 recommend that patients reduce the dose of one med while they transition on to a new one, for the reason u mentioned -- no lapse. docs call this a "cross taper". Unclear how long it will take for zoloft (sertraline) to work. some pts feel btr in just a few days, while others may need up to 8 wks on the right dose. ring ur doc to discuss :)