He’s also looking for side effects. In humans, rapamycin has been known to cause mouth sores, a spike in triglycerides, and slow wound healing; in mice, some evidence suggests it increases the risk of cataracts. Of the 25 dogs enrolled so far, Kaeberlein has yet to see any significant side effects; meanwhile the data on cardiac function, he says, is encouraging. Next Kaeberlein hopes to win approval for a much larger, three-to-five-year trial involving hundreds of dogs. It would be statistically significant and examine many more metrics, such as cancer rates, cognitive function, immune function, kidney function, and mobility.