I am bothered by the lack of precision in the question.
How much is "a glass"? Eight ounces, twelve ounces, a British pint, a liter...?
How much is a "drop"? How concentrated is the poison?
Every day we ingest small amounts (less-than-lethal quantities) of a variety of "poisons." Some are drugs that keep us alive by keeping more vicious threats at bay. Some are environmental, like chelated mercury. Just today the actor Jeremy Piven was said to have incurred mercury poisoning by eating sushi TOO FREQUENTLY (twice a day, they say, for a long long time). Yet when I enjoy sushi, I am more concerned about the parasites it might contain than the prospect of becoming a human thermometer.
But all this is a digression. The point is that TURN-ABOUT IS FAIR PLAY, and it is easy to show that the WATCHTOWER "cup" has more than one "drop of poison" in it.
I too, am interested in poisons, by the way.