Cerberus II

From Memory Alpha, the free Star Trek reference

Cerberus II is a planet in the Cerberus system, near Federation space. By the 24th century, the natives of Cerberus II had developed a medical process for rejuvenating aged bodies called the Cerebusian rejuvenator. The process was quite dangerous and painful and was not usually administered to aliens. However, in 2364, Starfleet Admiral Mark Jameson, who years before had negotiated a treaty on the planet, was able to obtain the process as a treatment for Iverson's Disease.

The process consists of herb and drug combinations that are self-administered over a period of two years and the reaction to the treatment depends on the individual being treated, specifically their DNA. Jameson broke the treatment procedure and administered the entire dosage at once, a move that ultimately caused his health to deteriorate even more rapidly. (TNG: "Too Short a Season")