Well, real simply, removing the Ativan cold turkey is probably what triggered her spikes in anxiety, which in turn kicked off a round of panic. In fact, going cold turkey off of ANY benzodiazepine (the family to which Ativan belongs) is not only only likely to induce anxiety, but can actually be dangerous physically - one needs to wean off of those medicines rather than just quit.
As for the SSRIs (Lexapro, Zoloft), they are really quite similar in their effect, and should not have created too much of an issue to move from one to the other. Of course, we are all different in our chemistry so may be affected differently... but this is less likely than the Ativan.
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