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drugs in body

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  • 10-05-2010 2:44am
    #1
    WalterMitty
    Registered Users Posts: 1,582 ✭✭✭


    So say a medicine is given to a person and they increase in weight susbtantially say50%. Will a medicine become less effective as it is more distibuted around the tissues? Or does it depend on type of tissue gained (muscle v fat) and does the sketeton store drugs and their metabolites? I know in specific case blood tests could assertain this but in general what are the variables involved?
    Cheers from a non scientist.


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    #2
    Capt'n Midnight
    Moderators, Recreation & Hobbies Moderators, Science, Health & Environment Moderators, Technology & Internet Moderators Posts: 90,760 Mod ✭✭✭✭Capt'n Midnight


    it depends on lots of things , you also have to factor in how fast the person can now metabolise or excrete the medicine

    as an example
    Vitamin's B and C are water soluble and aren't stored in the body so you need to take regularly but it's hard to overdose without trying.

    Vitamin A, D,E are fat soluble and are stored in the body. Eating polar bear liver can be fatal since it's got so much vitamin A.


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    #3
    SomeDose
    Registered Users Posts: 252 ✭✭SomeDose


    WalterMitty wrote: »
    So say a medicine is given to a person and they increase in weight susbtantially say50%. Will a medicine become less effective as it is more distibuted around the tissues? Or does it depend on type of tissue gained (muscle v fat) and does the sketeton store drugs and their metabolites? I know in specific case blood tests could assertain this but in general what are the variables involved?
    Cheers from a non scientist.

    For some medicines, yes the effect is very much dependent on the weight and body composition of the patient. This is usually only relevant for non-oral drugs in acute scenarios i.e. those administered by the intravenous and subcutaneous routes for example. In simple terms, the body consists of multiple "compartments" (blood, fat, muscle etc) into which a drug can distribute, and these compartments influence a drug's metabolism and magnitude & duration of effect. Some drugs just hang about in the plasma, whilst others can enter into fat or bind to tissues, proteins etc.

    A good example here would be gentamicin, an IV antibiotic. We know that it has pretty rubbish tissue penetration and is hydrophillic (attracted to water rather than fat), therefore it will not distribute into fatty tissues or across cell membranes once injected - basically it just hangs about in the one compartment (plasma). So not only is it dosed according to bodyweight, but it must also be dosed according to ideal (lean) bodyweight for patients with significant levels of bodyfat. This also explains why we use a higher dose per kg of gentamicin in neonates than in adults, as water makes up a larger proportion of a neonate's bodyweight therefore they have a larger "compartment" into which the drug may distribute.

    In practice, nearly all "everyday" oral medicines are given at a fixed doses and titrated against their desired therapeutic effect e.g. antihypertensive drugs are gradually increased in dose to achieve a target BP, oral antidiabetic drugs titrated up to achieve good control of blood sugars etc. This makes sense since it would be impossible to manufacture chronic oral drugs according to every person's bodyweight.


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