Something that’s always bothered the heck out of me. The labels always provide a does for adults and children 12y and older. So a 180 lb man takes the same dose as a 90 lb child? How about a 270 lb man?
This is something that has always bothered me as well, since I’m a very skinny adult (the same weight as your average 12-year-old, in fact) and find that I’m sensitive to many medications. I usually dose myself as though I was a child 12 or under, or cut pills in half for best results.
I also have an issue with the fact that medications like birth control pills are one-dose-fits all. And to a lesser extent, psychiatric medications (there are different dosing options available for these at least, but doctors don’t seem to take body size or weight into account on initial prescription, which has personally caused me problems). I can’t help but think that there is no way that a medication that is prescribed in the same dose to a 95-lb adult and a 350-lb adult is serving both of them as intended.
You’re not wrong, the dosage is based on an average person and will work for an average range of weights. If you’re out of the average as the 350lb person would be you could ask your doctor and/or pharmacist about it and they will know.
I believe asprin dosages are titrated upwards with weight.
I suppose there would be stability issues for some meds, but it would make sense to me to have medication in liquid form so that 1 ml (or other convenient measure) contains a dose for a 100 lb. adult. Then a Dr. or pharmacist could prescribe an accurate dose tailored for the person.
Good question. In general, a person weighing 50kg (110 lbs.) or more is considered an adult for dosing purposes, and is prescribed the ‘adult dose’. For a person weighing less than 50kg, the dose is titrated down based on their weight.
There is also a maximum recommended dose that must be considered. A guy weighing 700 lbs. is not going to be advised to take 15 Tylenol capsules.
mmm
Really depends on the medication. If it’s a medication that effects a particular area of the body the total weight of the person doesn’t make a difference. A 90 pound person and a 300 pound person still have a 3 pound brain.
That’s what I was going to say.
Isn’t this one of the several reasons why certain meds have to be by prescription?
I see your point, but isn’t the way an oral medication (or any drug) is transported to whatever specific area it is designed to act upon, how long it takes your body to process a dose, etc affected by body size and composition also? I may have close to the same brain weight as a 250-lb person but I have 140 less lbs of fat, muscle, blood, veins etc and as someone with a tiny torso, I’m sure my organs are smaller than average size as well.
Plus I have heard that many drugs are specifically ‘attracted’ to body fat. So not only one’s total weight, but also body fat percentage, will determine your reaction. Someone who is 130 lbs with 10% body fat is a different animal than someone who is also 130 lbs but has 30% body fat.
Just because one is 350 lbs doesn’t mean their nervous system will have twice as many opiate receptors as a 175 lb person. So just scaling the dose of a long-acting opiate narcotic painkiller up for the extra weight could end up causing an overdose.
Having said that, it’s a complex issue, depending on the fat distribution characteristics of the drug, how fast it’s cleared, how tightly it bonds to a receptor (if it is such a drug), its exact mechanism of action and elimination, the bioactivity of its metabolites, habituation to similar drugs, liver metabolic activity, etc, etc, etc.
Most physicians have access to references that will give the dosage by weight (mg/kg generally) when its applicable, and give information about situations where the dose needs to be increased/decreased.
Thanks as usual for your expertise. Why does it matter that it is long-acting?
Regards,
Shodan
You need to combine the possibility of a long acting med with the possibility that the user is an abuser… in some cases, a single pill misused can cause an od, where another user may need several times the dose due to habituation.
There is no top level of opiate use, so someone who has been a pain management victim/user for a decade may be taking several hundred times the dose that a new patient with the same profile and pain level and weight level would be taking. If the patient is transferring into a new practice, all of this needs to be taken into account.
Other drugs have different levels of tolerance as well- and some have no direct od, like tylenol, which will kill you through liver toxicity over a certain level (see max levels noted for 700lb man upthread), while others will simply not bind to receptors over a certain limit, so larger doses are less than helpful.
Many thanks, that’s kind of what I was wondering about.
The public has access to this information too. It’s just that most patients don’t know about it, nor are they particularly trained to use the information once they have it. I think you can ask a pharmacist for the prescription information/health care professionals monograph, or often just go to the drug website and look it up yourself. Using the first random drug that came to mind, I googled Zoloft, clicked on “Full Prescribing Information” to get this pdfwhich covers the pharmacokinetics, metabolism and other information that may be used to determine whether the drug is appropriate for a patient.
At the other end of the alphabet, and for an OTC, advil.com has a link at the top that reads “healthcare professionals - click here”. You then get to confirm you’re a healthcare professional after reading a disclaimer that it isn’t for the general public, but you can still get to the Advil Aidewebsite and read all about the drug. Hereis a dosing chart.
It’s an issue with short-acting opiates too, but less of one, due to the fact that receptor sites for the opiates take a while longer to be filled in larger people, due to the volume of distribution of the opiate in the blood and the time it takes to cross the blood-brain barrier. So short-acting opiate dosages would indeed need to be a bit larger to get the same pain relief in a larger person. And with them leaving the receptor sites in a short time, overall the number of opiate receptors activated over time in a larger person could be less.
In the long-acting opiate situation, it may take a while longer for the medication to take full effect in a larger person, but once the opiates have reached their target, they’ll persist there and the overall number of receptors filled over time will be pretty much the same in smaller and larger folks.
All of this just points to all the different subtle things that come into play when adding drugs to one’s system. And that’s without even mentioning whether or not the microsomal enzyme oxidizing system has been induced! 
When I hurt my knee playing soccer (not serious) the doctor I saw at the time asked me how much I weighed. When I told him, he thought for a minute, then told me to take 800 mg of OTC Ibuprofen twice a day, which is more than the label recommended dose.
I assume this means that ibuprofen scales with body size, but of course IANAD so don’t take my word for it.
Well, my doctor has told me the same thing, and I’m going to guess I weigh more than you. Isn’t 800mg just prescription strength?
Not even going to pretend to be an expert, but no. Looking at Advil’s site, the standard dosage for an adult is 200 mg. A double dose can be used for an adult (400 mg) but the website has cautions about it.
I’m a big guy too - not huge, but above average. Not sure where you are, but ask a proper doctor.
They just don’t want you self-dosing that high without consulting a doctor, hence the labeling on OTC preparations, but there are in fact ibuprofen products which are 800 mg per pill. The maximum daily dose is 3200 mg/day, or 800 mg q6h. I am an average-sized woman (currently overweight, but I haven’t always been), and I have taken this much ibuprofen several times at the advice of a doctor and/or pharmacist.
Rxlist
As an aside, I don’t know if this exists in human med, but in vet med there are occasionally drugs which are dosed to the patient’s ideal weight rather than actual weight if the patient is obese, the idea being that the extra adipose tissue does not have any effect on the pharmacokinetics of that drug. I think this is mostly for drugs with narrow therapeutic margins. Vet med in general almost always uses weight-based dosages (rounded to the nearest convenient dosing unit), maybe because species such as dogs and horses have far more variability than humans in lean body mass.
There are also some drugs, such as some chemotherapy drugs, which are traditionally dosed in both human and vet med according to body surface area (as estimated with a formula), not mass.
Why dosed according to body surface area?