We see in the beginning of season 3 of Arrow that Sara dies (at the hands of a drugged Thea shooting arrows at Sara).
However, we also see later in the season that Roy "dies" fairly convincingly and is even legally/medically deemed dead. But we later find out that his "killer" was a specialist at faking realistic deaths, so it seems like no matter how realistic the death seems, it is possible for characters to live.
So that begs the question, could Merlyn have given Thea a drug that could allow her could "kill" Sara convincingly enough, but not actually kill her? Does there exist such a drug in the DC Television Universe?