It's not clear to me from this article whether he was currently taking it. It certainly sounds like bad news:

From, side effects of the drug Fanapt:
Psychiatric side effects including restlessness, aggression, and delusion have been reported frequently. Hostility, decreased libido, paranoia, anorgasmia, confusional state, mania, catatonia, mood swings, panic attack, obsessive-compulsive disorder, bulimia nervosa, delirium, polydipsia psychogenic, impulse-control disorder, and major depression have been reported infrequently.
Adam Lanza Taking Antipsychotics - Business Insider

Why am I not surprised by this? I was actually waiting to see pharmaceuticals turn up in the news reports.

Random killings seem not to be a new phenomenon. Whether they occurred as often in the past is another question. And if the truth were known, I think we might be surprised at how often pharmaceuticals are involved in current murder-sprees. I don't mean to say that they would be "the cause" but that they would be there as a factor.

The killer was also said to be a vegan, which can hardly have helped his mental stability.