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    Can someone please explain exactly what the half life of a drug means and how it shows with it's effects?

    I did this in biology...it's the time taken for the drug to get to half the level in your body but what exactly does that mean and how does it show with how it affects the person?

    An example: if someone stopped taking quetiapine, the half life is 6 hours (parent compound); 12 hours (active metabolite) (thank you wikipedia)....WTF does that mean? Does that mean after 24 hours it's at a 1/4 of the level so taking 100mg 24 hours later would be at 25mg then?

    And how long would it take to leave the body entirely, is there a mg level where it's basically zero? Because half goes to half infinity there's no zero.

    Very confused by all this. :confused:
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    (Original post by Anonymous)
    Can someone please explain exactly what the half life of a drug means and how it shows with it's effects?

    I did this in biology...it's the time taken for the drug to get to half the level in your body but what exactly does that mean and how does it show with how it affects the person?

    An example: if someone stopped taking quetiapine, the half life is 6 hours (parent compound); 12 hours (active metabolite) (thank you wikipedia)....WTF does that mean? Does that mean after 24 hours it's at a 1/4 of the level so taking 100mg 24 hours later would be at 25mg then?

    And how long would it take to leave the body entirely, is there a mg level where it's basically zero? Because half goes to half infinity there's no zero.

    Very confused by all this. :confused:
    1. I have no idea why you are anon.

    2. This should be in a different forum

    Onto the question...a drug with a longer half-life will remain active in the body for longer. Sometimes this is great, a long half-life means less frequent dosage. But sometimes it isn't, any occurent side effects are prolonged.

    The wiki thing means that the drug will stay as you consumed it with a half-life of 6hrs. But when it is changed in the body to the form that will actually work as a medicine, the drug will be present in this form with a half-life of 12hrs (concentration of the ACTIVE drug will halve in 12 hours).

    Whereas lesser than half is concerned, this is different for each drug, as every drug has it's own properties. But a shorter half life is a good indicator that a drug will completely clear sooner. It can't be said that if the half-life is 6hrs, the drug will be completely cleared from circulation, because there may well be a 'curved (graph measurement) response'.
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    In pharmacokinetics a drug is considered to be effectively out of your system in 5 half lives. At what point a drug ceases to have an effect on a person depends on the concentration needed to have an effect, often referred to as the therapeutic window.

    In your example when you take quetiapine it will be distributed around the body, not all of it will necessarily be present in the plasma. After 24hrs the concentration of actual quetiapine in the system will have fallen to 1/16th of the original (quetiapine half life 6hrs). Quetiapine is however changed by the body to an active metabolite. In 24 hrs the concentration of metabolite with have fallen to 1/4 of it's original concentration.
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    (Original post by ash92:))
    ..
    (Original post by shonaT)
    ..
    Are either of you able to help me with the question in this thread relating to HL?

    http://www.thestudentroom.co.uk/show....php?t=2258309
 
 
 
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