I'm not quite sure about that.
In the same situation I would figure that anything that might change his mental state might be a good gamble. After all, she is tied to the bed by a guy who just killed 4 people so it's a good bet that his current mental state isn't too conducive to her getting out of the situation alive. Since she was a user herself, I am guessing that she could guess what the effect would be.
That statement makes the assumption that the drug would have the same effect on all people. It doesn’t. In fact, for all she knew the influence of the drug is what made him homocidal.