Humans grow two sets of teeth in a lifetime. But tucked inside the jaw are tiny residual tooth buds that never develop because a protein called USAG-1 acts like a molecular off-switch for tooth growth. Block USAG-1, and the body might reactivate those dormant buds on its own. Researchers at Kyoto University Hospital, led by Dr. Katsu Takahashi, have been working on this for years. They first found that mice engineered without the USAG-1 gene grew extra teeth. Later animal studies showed that a single antibody injection could grow fully functional teeth without serious safety issues. The experimental drug is called TRG-035, a humanized anti-USAG-1 antibody developed by Toregem BioPharma. The first human trial began in October 2024 and enrolled 30 men aged 30 to 64, each missing at least one molar. The Phase I study was primarily a safety trial — a randomized, double-blind, placebo-controlled dose-escalation study. According to Toregem, Phase I confirmed the drug's safety, though detailed results haven't been made public yet. Researchers have noted that actual tooth regrowth in those adult participants was unlikely, since the volunteers were primarily establishing a safety profile before testing expands. The next stage is where it gets more interesting. A Phase IIa trial is planned for 24 children aged 2 to 7 with severe congenital tooth agenesis — a condition where teeth never fully develop. Missing six or more teeth from birth affects roughly one in a thousand people, causing severe chewing difficulties and significant social stigma. The trial will be open-label and non-comparative, moving testing from healthy adults into the patients the drug is actually intended to treat. There are still open questions. Experts have raised concerns about whether regrown teeth will align properly and connect correctly to nerves and blood vessels. If the trials succeed, Toregem aims for commercial availability around 2030, with early estimates placing costs at around $11,000 per tooth. It's experimental. It's early. But the idea that human jaws might hold a dormant third set of teeth waiting for a chemical wake-up call is the kind of thing that sounds like science fiction until someone actually tries it. Sounds like of one those medical breakthroughs we hear about it once, and then after several years we keep wondering what the hell happened to that. Like the UK researchers who find 100% effective hair regrowth medication every odd year. It’s a sensational headline, but there’s a LOT to be ironed out before it can even begin to think about something like a clinical trial. Human babies are born with two sets of teeth in their jaws already developed. We don’t know if fully-formed and calcified adult jaws can support growing new teeth or if they’ll align or erupt correctly. We don’t know if adult bodies can form new veins and nerves to support new teeth. We have no idea if tooth buds can even be reactivated after being switched off or if they just completely disappear after childhood. Once the gene is switched on, how do we make sure it switches off again at the right time? We don’t want patients growing endless teeth like sharks. Stimulating any kind of growth or regrowth always runs the risk of causing cancer. That is a major, intractable hurdle for the entire field of regenerative medicine. These kinds of hurdles take years of research and millions of dollars, and at any point the entire project might just explode if something doesn’t work or causes super mega cancer.