In physics , a fixed time required for half the radioactive nuclei in a substance to decay. Half-lives of radioactive substances can range from fractions of a second to billions of years, and they are always the same for a given nucleus, regardless of temperature or other conditions. If an object contains a pound of a radioactive substance with a half-life of fifty years, at the end of that time there will be half a pound of the radioactive substance left undecayed in the object. After another fifty years, a quarter-pound will be left undecayed, and so on.
The longer a drug’s half-life is, the longer it takes for the body to eliminate a single dose. This is medically significant in many instances, because drugs can interact with one another, and because some can influence the success of a surgical operation. For example, the blood thinner warfarin is prescribed to people who are at risk of developing blood clots. A person who is preparing to undergo a surgical procedure must stop taking the drug several days in advance, to eliminate it from the body and reduce the risk of excessive bleeding during the operation.