She should have waited (Spoiler Alert)

Spoiler Alert (after all, this movie has only been out FIFTY-FOUR YEARS)

Dr. Cartwright should have waited longer before poisoning Tunga Kahn and herself. There is no indication in the movie that more than a few minutes have elapsed since the other six white women and the Asian woman left in the wagon. It's likely that one of Tunga Kahn's minions would walk in at any time, find them dead, see the empty poison bottle on the dresser, and send riders after the women to bring them back for an unspeakably barbaric fate. If she had waited hours or a couple of days until she was certain they were out of harm's way, Cartwright would have had to endure at least one disgusting sex session with him, but she would have ensured their escape.