Prescribing drugs
April 5, 2010 5:56 AM Subscribe
How do doctors decide what dosage of medicine to give a patient?
Assume the patient is an adult. (I understand children metabolize drugs differently than adults.)
If the patient is a 100 pound woman, does the difference in her body mass from, say, a 200 pound man or a 300 pound offensive lineman, affect the doctor's decision in dosages?
Or, is this specific to the drug itself and not the patient's body mass?
(I was prescribed some drugs last week and suddenly thought of this question. I'm not really concerned about the dosage prescribed to me; I was just thinking about it more generally.)
Assume the patient is an adult. (I understand children metabolize drugs differently than adults.)
If the patient is a 100 pound woman, does the difference in her body mass from, say, a 200 pound man or a 300 pound offensive lineman, affect the doctor's decision in dosages?
Or, is this specific to the drug itself and not the patient's body mass?
(I was prescribed some drugs last week and suddenly thought of this question. I'm not really concerned about the dosage prescribed to me; I was just thinking about it more generally.)
Usually, the same dosage is prescribed for every size of adult, although in some cases this results in inadequate medication levels for heavier people.
posted by Ery at 6:17 AM on April 5, 2010
posted by Ery at 6:17 AM on April 5, 2010
It's specific to the drug. My husband and I were both prescribed the same dose of a painkiller, even though I'm 95 lbs and he's around 170.
Drugs like SSRIs are often ramped up, so a person who's just started taking Zoloft will be on a different dosage than someone who's been taking it for a year.
posted by desjardins at 6:18 AM on April 5, 2010
Drugs like SSRIs are often ramped up, so a person who's just started taking Zoloft will be on a different dosage than someone who's been taking it for a year.
posted by desjardins at 6:18 AM on April 5, 2010
A lot of drugs are prescribed based on "body surface area," which is in turn estimated using a quadratic formula based on weight.
posted by miyabo at 6:24 AM on April 5, 2010
posted by miyabo at 6:24 AM on April 5, 2010
And some drugs have really wonky metabolism and are adjusted based on regular blood tests. Warfarin, for example.
posted by SLC Mom at 7:00 AM on April 5, 2010
posted by SLC Mom at 7:00 AM on April 5, 2010
It took years, but it eventually dawned on me that the reason a lot of OTC drugs say "1-2 pills" or whatever is because smaller people frequently need only one and larger people may need two.
This also solved why NyQuil gave me such a dire hangover, and I finally started taking the smaller dose. Duh!
Many children's drugs are prescribed by weight.
Some drugs affect men and women differently, and may affect people of different genetic backgrounds differently (which in practical terms means using race is a rough guideline for dosing). But most drugs dosing guidelines are for a "standard-weight white male" which means women and minorities and very large or small people may suffer more side effects or get less proper effect from the medicine. Every now and then there'll be a big story in the media about diversity in drug dosing.
posted by Eyebrows McGee at 7:09 AM on April 5, 2010
This also solved why NyQuil gave me such a dire hangover, and I finally started taking the smaller dose. Duh!
Many children's drugs are prescribed by weight.
Some drugs affect men and women differently, and may affect people of different genetic backgrounds differently (which in practical terms means using race is a rough guideline for dosing). But most drugs dosing guidelines are for a "standard-weight white male" which means women and minorities and very large or small people may suffer more side effects or get less proper effect from the medicine. Every now and then there'll be a big story in the media about diversity in drug dosing.
posted by Eyebrows McGee at 7:09 AM on April 5, 2010
Oh, I have another one for you: I'm a redhead, which means anesthesiologists give me more anesthesia than non-redheads. For decades it was just a piece of anesthesiologist "folk wisdom" but now there's some evidence that the melanocortin-1 gene which controls red pigment in hair also affects reaction to anesthesia (redheads need more) and sensation of pain. So there's a very unusual dosing criterion!
posted by Eyebrows McGee at 7:13 AM on April 5, 2010 [1 favorite]
posted by Eyebrows McGee at 7:13 AM on April 5, 2010 [1 favorite]
I have a friend who is a pharmacologist and she says that race often affects absorption rates of a particular medication. Some people are very sensitive to X drugs in low dosages, some people aren't.
posted by anniecat at 7:24 AM on April 5, 2010
posted by anniecat at 7:24 AM on April 5, 2010
So here's a good one for you, Eyebrows McGee -- I woke up on the table during surgery (ugh), and while a very unnatural redhead, I've definitely got "redhead genes" in the family -- I wonder if you can have the melanocortin-1 gene (and the related effects) even if you personally don't have (real) red hair. Anyone know?
Then again, maybe the anesthesiologist was just an idiot. Hard to say. (This was the hospital that managed to lose my blood test results that were taken less than a week before the surgery, after all).
Also -- some drugs work better on certain people than others do -- my father and I both might as well not even bother taking aspirin/etc, whereas the stronger codeine-related drugs work just fine. And we have the same (weird) migraine triggers, too...
posted by bitter-girl.com at 9:26 AM on April 5, 2010
Then again, maybe the anesthesiologist was just an idiot. Hard to say. (This was the hospital that managed to lose my blood test results that were taken less than a week before the surgery, after all).
Also -- some drugs work better on certain people than others do -- my father and I both might as well not even bother taking aspirin/etc, whereas the stronger codeine-related drugs work just fine. And we have the same (weird) migraine triggers, too...
posted by bitter-girl.com at 9:26 AM on April 5, 2010
The aim with any drug dosage is to achieve a certain concentration in the body (typically measured in mg of drug/mL of serum). Weight of the subject can be used as general guide for dosage, but all the cool kids are using body surface area (BSA) for determining dose sizes.
As for how anyone even knows where start with a dosage, this is one of the key things determined during Phase I and II drug testing. Any drug trial worth its funding will include dose-escalation (to figure out what concentration of drug will work, but not kill you) and pharmacokinetic testing (to figure out where and for how long a drug stays in your body). All this information is combined to establish guidelines for for what the ideal concentration of the drug is, which can then be combined with information on your BSA to determine your individual dosage. Here's an overly complicated dose calculator for carboplatin, a chemotherapy drug.
As for the effects of an individual's genetic make-up on a drug, there's a whole slew of pharmacogenetic tests used to examine that.
posted by Panjandrum at 11:13 AM on April 5, 2010 [1 favorite]
As for how anyone even knows where start with a dosage, this is one of the key things determined during Phase I and II drug testing. Any drug trial worth its funding will include dose-escalation (to figure out what concentration of drug will work, but not kill you) and pharmacokinetic testing (to figure out where and for how long a drug stays in your body). All this information is combined to establish guidelines for for what the ideal concentration of the drug is, which can then be combined with information on your BSA to determine your individual dosage. Here's an overly complicated dose calculator for carboplatin, a chemotherapy drug.
As for the effects of an individual's genetic make-up on a drug, there's a whole slew of pharmacogenetic tests used to examine that.
posted by Panjandrum at 11:13 AM on April 5, 2010 [1 favorite]
This thread is closed to new comments.
In many cases, the drug's action isn't actually from metabolic processes anyways. Antibiotics can be recovered in useful form from urine, and NSAIDs are metabolized by the liver after their effects. So body mass won't necessarily have all that much of an effect on dosage a lot of the time.
posted by valkyryn at 6:02 AM on April 5, 2010