Common Crazy Med Crap Index
Metabolism & Elimination | Pharmacology Index

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  1. 1. References

Why is knowing about absorption, distribution, bioavailability and other obscure crap important?

  • Absorption can be affected by food. Knowing how much of a difference it makes when you take a med after eating vs. on an empty stomach may not seem like a big deal, until you can’t eat much of anything.
    • Say if you got very sick
    • Or had gastric bypass surgery
    • Or the med makes you want to puke when you take it on an empty stomach
  • Bioavailability and distribution explain why some meds are vastly more potent than others (e.g. Paxil and Prozac), but there’s not a lot of difference in their dosages.
  • Bioavailability factors determine if a generic medication is functionally equivalent to the original brand-name drug. You can’t call bullshit on the generic’s manufacturer if you don’t understand the numbers, because the FDA claims any difference you experience is imaginary or coincidental.

Bioavailability made simple: practically all injected drugs are about, if not absolutely 100% bioavailable.
As most of the drugs we deal with here are taken orally (even if they do have an injectable version available), the simple version of bioavailability usually isn’t an option. So what do you need to know about bioavailability? Unless you’re studying to be a pharmacist or doctor, or you just like to learn everything you can about a med, you don’t need to know any of this.
Except when there’s a problem with a generic medication, and you want to figure out what the hell is wrong14. Not that you could do anything about it, or that the data on the generic medication are readily available, but it’s nice to know that the problem isn’t in your head. Rather the generic med in question doesn’t act the same way as the brand or another generic somewhere between your stomach and your brain. The most notorious example of this to date is Teva’s Budeprion XL version of Wellbutrin XL. We go into great detail about it in the section on branded vs. generic drugs, with an example of how these data are used.
The plasma half-life is the same as above and repeated here just to make it easier to perform any calculations you may want to do.


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However, the FDA requires only a single, frequently sub-therapeutic dose taken by healthy volunteers to test pharmacokinetic data, including bioavailability and bioequivalence factors. Until recently all pharmacokinetic data for a drug were based on a single study done on fewer than 20 healthy volunteers, who were mostly, if not all white males between the ages of 20 and 50 willing to take recently developed drugs because they need the money. Those guys took a single dose, which was often lower than the typical starting dosage, on an empty stomach, first thing in the morning. The only way to tell if the data is from the real world (e.g. from people with the condition the med treats who are in stage III clinical trials) is to look at the pharmacokinetics section of a drug’s Full US PI sheet, or some of the better overseas equivalents. If it doesn’t tell you how many people participated with demographic data, assume it was a handful of healthy white guys desperate for cash.
You can probably tell how those crappy studies create a big problem with pharmacokinetic data.

Fortunately more and more pioneering pharmaceutical companies (the ones that make the original, brand-name drugs) are using far more people in their studies, including those with the conditions as well as healthy volunteers to determine all PK data, they are not required to do so. By the US FDA. Other countries have different standards, and that forces big pharma to provide better PK data.
But unless a it’s a completely new drug that has been approved recently, or an existing drug that has been recently approved for a very different application, including overseas approvals, the PK data are probably from a dozen white guys who needed the money.



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Common Crazy Med Crap Index
Metabolism & Elimination | Pharmacology Index


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1.  References

  1. Stahl’s Essential Psychopharmacology: Neuroscientific Basis and Practical Applications (Essential Psychopharmacology Series) Third edition by Stephen M. Stahl © 2008 Published by Cambridge University Press.
  2. Primer of Drug Action 12th edition by Robert M. Julien Ph.D., Claire D. Advokat, Joseph Comaty © 2011 Published by Worth Publishers.
  3. Genetic Basis of Drug Metabolism

11 See footnote 2 above. The good news is generic manufacturers are required to be at least 90% sure of everything being within the 80–125% range.12 If we see any evidence of a generic manufacturer ever doing more to establish the 80–125% bioequivalence standard than giving one small dose of the med to the 20 white guys who have already sold their monthly allotment of plasma, we’ll note it.

12 I know that’s not what a 90% confidence interval really means, but sometimes that’s what it seems like. Which is why we list all known generics (if any are available) that have been independently verified to meet the 80–125% standard on {{$$brandname}}’s Expanded Brand & Generic page.


Pharmacokinetics 101 - Absorption, Distribution & Bioavailability by Jerod Poore is copyright © 2012 Jerod Poore

Last modified on Sunday, 14 September, 2014 at 18:28:32 by JerodPoorePage Author: Jerod PooreDate created: 28 April, 2012

All drug names are the trademarks of someone else. Look on the appropriate PI sheets or ask Google who the owners are. The way pharmaceutical companies buy each other and swap products like Monopoly™ real estate, the ownership of any trademarks may have changed without my noticing.





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