This was a 72-week trial of tirzepatide, one of the weight loss injections, in 2,539 adults with obesity who did not have diabetes. People were assigned at random to a placebo or to one of three doses, and neither they nor their doctors knew which they were getting. The trial was built to measure weight loss, and it measured a great deal of it. What matters here is something it recorded along the way rather than something it set out to study.

Hair loss was reported by about 1 percent of the people on placebo, 6 out of 643. In the three tirzepatide groups it was reported by 5.1, 4.9 and 5.7 percent, so roughly five times as often. Read that number for what it is. Hair loss was not an outcome this trial was designed to measure. It was logged when participants happened to mention it, with no dermatologist examining anyone, no scalp imaging, no pull tests and no bloodwork to sort out what kind of hair loss it was or what was driving it. There is no follow-up showing whether it recovered. The trial was funded by Eli Lilly, who make the drug, and five of the eleven authors are Lilly employees. On this particular question that cuts an unusual direction, since a manufacturer has no reason to surface a side effect nobody asked about.

Here's the study: Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide Once Weekly for the Treatment of Obesity. New England Journal of Medicine, 2022;387(3):205-216. Read it here

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