Non-variable recommended drug dosage
April 14, 2020 6:57 AM   Subscribe

The recommended daily dosage of cetirizine (Zyrtec) is 10mg, whether you're a 40 lb six-year-old or a 300 lb adult. How can this be? Does it really have a similar effect?
posted by East Manitoba Regional Junior Kabaddi Champion '94 to Health & Fitness (6 answers total) 1 user marked this as a favorite
 
Hey! I (kind of) know this one. I have read the dosage studies for cetirizine -- basically, 10 mg worked better than 5 mg, but 15 mg had no benefit over 10 mg.

Now, one can assume these were all done with your standard population that does guinea pig work, so it's probably very biased toward youngish white men who had time to take part in a medical study. Meaning, you are probably right that dosage does or should vary by body weight. With a very safe OTC drug like cetirizine, though, that's less important than it would be for a drug with a very narrow therapeutic window.
posted by fiercecupcake at 7:10 AM on April 14, 2020 [2 favorites]


About 15 years ago, I read an interesting book on this subject. The book is called Over Dose: The Case Against the Drug Companies, by Jay Cohen, MD. The author argues that drug companies like to keep dosages simple, so they create easy, one-size-fits all dosages that are unrealistically high for many patients. When the higher dosages are used in clinical trials, they can result in better effectiveness statistics. However, patients also experience side-effects that probably could have been avoided, if the drug had been dosed at a more-reasonable level.

Cohen's book was first published in 2004, but I think the general principles still hold up well today.
posted by alex1965 at 7:12 AM on April 14, 2020 [4 favorites]


I am a study of 1 and my experience with ceterizine is that 5mg works for me. 10mg makes me sleepy. I'm new to seasonal pollen allergies so I've been experimenting. I am now experimenting with if I can take it every other day. I weigh around 150.

I have some children's regular cetirizine, the dose for under 2-12 year olds is 5 mg once a day or 2.5 mg twice a day on my bottle.
posted by RoadScholar at 7:20 AM on April 14, 2020 [2 favorites]


It looks like adjusting dosage based on weight in adults needs more study, but this article says

"Drug clearance represents the functional capacity of the body to metabolise and excrete a drug. Clearance is correlated to lean rather than adipose weight as adipose tissue has little metabolic activity. As clearance determines a drug’s maintenance dose, clinicians should consider how lean body weight, rather than total body weight, impacts dosing."

So for some drugs total body mass isn't the major factor affecting how a drug is processed in the body.
posted by needs more cowbell at 7:40 AM on April 14, 2020 [3 favorites]


The science (summary) of a single dose of 10 mg of 14C-cetirizine dihydrochloride:
  • cetirizine is rapidly absorbed - most within one hour
  • half of the drug was eliminated within 7.4 hours
  • 60% of the cetrizine is eliminated within 24-hour in urine.
  • a total of 80% of the cetrizine was recovered as the unchanged drug over the 5 day study.
So the dosage is the same regardless of body weight because, essentially, everybody is taking much more than their body actually utilizes. It's also science that people are terrible at drug schedules, so one of the primary concerns is to simplify that critical step. Once a day is easier to get compliance than twice a day.

Personal advice: if you find your allergy medication makes you drowsy try taking half doses more frequently, and use your phone to remind you. Also, find yourself an allergist who is a huge nerd about it, even if you can only call them.
posted by zenon at 8:59 AM on April 14, 2020 [6 favorites]


zenon's explanation is similar to the one I got from the vet, who prescribed cetirizine at a full dose 2x/day because canine elimination rates are higher than humans, and with the way histamines work in seasonal allergy scenarios it's almost pointless to only dose once a day because you don't get enough continuity.

After that I myself started taking 5mg morning and evening and I do feel like I'm getting better round-the-clock coverage that way.
posted by Lyn Never at 9:06 AM on April 14, 2020 [2 favorites]


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