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  1. #1
    farmer123 is offline Newbie 510 points
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    pharmacokinetics problem

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    Heya people,

    If you double the dose of a drug, its duration of action in the body is only increased by one half-life.

    I think I understand this concept, but in trying to answer the following question but I run into some weird conceptual problems.

    The question:


    The half-life of drugA is 8 hours. If a person takes 1000 mg of this drug it falls below its clinically effective range 12 hours after ingestion. In general, if the patient wants this drug to be effective for 36 hours, how much should the patient take?

    I know this seems like a simple problem but, I am not sure if I am doing it right.

    My calculations:

    halflife= 8 hrs
    1000mg drug effective for= 12 hours
    how much drug needed, in mass, to be effective for 36 hours?

    1000mg-->2000mg--->4000mg--->8000mg
    12hrs------>8hrs--------->8hrs------->8hrs

    So, if I double the drug dosage from 1000mg to 2000mg, I would only end up increasing the duration of action by one half life, which is 8 hours. If I double the dosage from 2000mg to 4000mg, it would again increase the duration by only 8 hours.

    Following the reasoning above, the person would need to take roughly 8000mg of drug in order for it be effective for 36 hours.

    This is what I think, but I don't think I am doing it right. I am not sure if I should use an equation since I can't find one that would be adequate for this question.

    Thoughts?

  2. #2
    CMD
    CMD is offline Junior Member 511 points
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    12hrs = 1 half life + 4hrs; so there still is 500mg of drug with 4 hours to go
    36hrs = 4 half lives (32hrs) + 4hrs ; the answer would be the amount of drug you need to give so that there still is 500mg with 4hrs more to go.

    Therefore the answer is 500mg * 2 * 2 * 2 * 2 = 8000 mg

  3. #3
    farmer123 is offline Newbie 510 points
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    Quote Originally Posted by CMD View Post
    12hrs = 1 half life + 4hrs; so there still is 500mg of drug with 4 hours to go
    36hrs = 4 half lives (32hrs) + 4hrs ; the answer would be the amount of drug you need to give so that there still is 500mg with 4hrs more to go.

    Therefore the answer is 500mg * 2 * 2 * 2 * 2 = 8000 mg
    Thanks CMD, that is the right answer.

    Alternatively, we can also use the equation C=Co x e^-kt

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