The botanical name for yaupon (Texas holly) is Ilex vomitoria. The "vomitoria" part refers to the belief that some Indian tribes used yaupon tea as an emetic. I certainly haven't had that experience.
In Stalking the Blue-eyed Scallop (1964), Euell Gibbons states that yaupon was not the plant used in the purging "black drink," but there has been some controversy in the literature. However, in Dangerous Plants, Snakes, Arthropods, and Marine Life (published by Drug Intelligence publications in 1978), Michael D. Ellis confirmed that the leaves contain no toxic substances, so the tea is safe.
However, the berries of yaupon and the other hollies are considered highly toxic to humans. As few as six berries have caused a child to vomit, and 20-30 berries have caused fatalities in children. So be careful with them.