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Brian - half-life is a concept used whenever there is a condition called "exponential decay". (This is the reverse of the "exponential growth" that makes population increases so worriesome.) With exponential decay, a substance disappears quickly at first; then, the rate of disappearance slows continuously. This contributes to the seriousness of nuclear contamination, since the darn stuff takes approximately forever to be really and truly gone. I'm a math teacher, not a medical professional, but I've seen lots of problems in math textbooks about the half-life of drugs. I _think_ the concept serves a purpose in pharmacology by facilitating a calculation of when all measurable traces of a drug will have disappeared from a patient's bloodstream, for example. It could also be used to calculate how long it will take for levels to fall below some specified minimum. It doesn't seem as if the notion of half-life, per se, would tell us how long a drug is effective (although the two concepts are related). If that was as clear as mud, pls ask questions! Mary cg for mom 75/6 [log in to unmask]