Yes, Zoloft is safe (an L2 drug). According to Medications and Mother's Milk by Thomas Hale PhD. (he is considered the leading researcher on medications and breastmilk) this is what Zoloft says:
Is has a long half life of 26 hours
It's peak plasma ratio (amount in your blood stream) is highest at 7 to 8 hours after taking it.
It has a 98% protein binding (PB) capacity - this is excellent since drugs with PB capacities over 90% make it harder to pass into the milk compartments.
It's molecular weight is 306 Daltons. Anything over 200 Daltons has trouble passing the mammary epithelium into the breast. In other world, the heavier the drug the less likely it passes. (I am on Remicade which weights 149,100!!!! daltons - needless to say, I nurse)
It has a very low Milk Plasma ration of 0.89 meaning that the ratio of the drug in your blood versus what appears in your milk is extremely low.
I can fax you the page from the book if you like. Facts like these based on research, physiology, chemistry and biology are always better than simple opinions.
In my opinion (and Hale's fact-based opinion) Zoloft is fine.
:)K.