Biology Stack Exchange is a question and answer site for biology researchers, academics, and students. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I'm really stuck on this question. I'm being asked to find the half life of a drug.

The Vd is 50L/kg. The clearance is 0.1 L/hr/kg. The patient weighs 70 kg. Equation is: T1/2 (halflife) = 0.693 * Vd / Cl. I keep getting 0.693 * 50 / 0.1 = 346.5 The right answer is 14.4 days but I can't figure it out. Thanks in advance

share|improve this question

You have done the calculation correctly it is just the answer you have is in hours, try dividing by 24.

A good method of checking answers is to use a trick which is horribly called "Dimensional Analysis". This basically means writing out your equations but use the units of each value rather than the value itself

so in this case you have T1/2 = Vd/Cl (removing the unitless 0.693)

the units for this are then ? = (L/kg)/(L/hr/kg)

in this case the litres and the kilos cancel to 1/(1/hr) or just hours, comparing this with the correct answer which is in days should then be a little more obvious

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.