Ex-Tend is an anti-agathic about which little is known. It appears to be a close relative of the drug FOREVER.
Although little is known about the mechanisms by which it works, it is known that it does not confer actual immortality. Most users are content to take it for now, and let it delay their aging until an even better anti-agathic is invented.
Ex-Tend exists in only a handful of closely-related alternate timelines or eigenstates.