topiramate’s half-life, elimination metabolism, drug-drug & drug-food interactions, how long until topiramate clears your system and more
Table of Contents (hide)
- 1. In a Nutshell
- 2. How Long Topamax (topiramate) Hangs Around
- 3. How topiramate is Metabolized, and What the Effects and Results of the Metabolism Are
- 4. Drug-Drug, Drug-Supplement, Drug-Food Interactions
- 5. topiramate Bioavailability and Bioequivalence Data
- 6. Comments and Miscellaneous PK Data:
- 7. References
Pharmacokinetics (PK)1 is what your body does to a drug, as well as what a drug does to your body in specific ways that usually don’t have much to do with most of the effects of most crazy meds. Unless you count side effects. Pharmacokinetics is a geek’s paradise. This is hardcore, detailed, technical information about a medication2 that explains things like drug-drug interactions, how long it takes to work, why you get certain side effects, and a lot more. Maybe even why some of the meds work the way they do after all. Explaining the various aspects of PK is so long and involved it requires its own page.
The numbers here are known as “plasma clearance,” which is how long it takes until there is no significant amount of a drug in your blood. I.e. you’re clear of it. How that is determined is very simple: the half-life times five.3
Half-life is the average time it takes for you to metabolize4 half of the drug’s active ingredient. If a drug has a half-life of around 24 hours and you take a dose of 100mg, you’ll have roughly the equivalent a 50mg dose after one day, a 25mg dose after two days, and so on.
The more-accurate complete clearance requires a hell of a lot more data, often, but not always including your weight and the dosage you’re taking. There may also be numerous passes required to fully metabolize a drug, and that applies even to medications that don’t have active metabolites! If we have the data, which we usually don’t, we’ll provide the best estimate we have for how long it really takes for a drug to clear your system. The data we need are found below in the bioavailability section and miscellaneous PK data because we rarely see them section. The formulae vary slightly depending on drug’s pharmacokinetics and how much we know about it. Dosage divided by AUC is the next step up in complexity. We can ratchet it up to Cp(t) = ((dosage * ka)/(V/F)*((ka - (CL/V)))*(e-(CL/V)*t - e-(CL/ka)*t), or even more complicated formulae. To make matters worse, practically all the data come from a small sample of
professional lab rats healthy volunteers.
The stupid thing is: complete clearance usually winds up being 2–5 days after plasma clearance no matter what5, but can take weeks. Sometimes a drug will clear from your brain and other organs before it clears from your blood. Prozac is one such drug, but with a half-life measured in days, that doesn’t surprise me too much.
For most people reading this site, elimination metabolism is what pharmacokinetics is all about. How a drug is metabolized in order to be eliminated / cleared from your body. If and how it is transformed into something else and what that is. How it affects the metabolism of other meds6 (see below). To what extent everything is involved. E.g. by the time your liver and kidneys are through with it, 97% of Paxil (paroxetine HCl) has been converted to inactive stuff that gets flushed down the toilet, whereas 70% of the Topamax (topiramate) you take is unchanged in your urine. See the section on CYP450 and UGT enzymes on the Pharmacokinetics page for more details.
The pill you take is not necessarily the drug that actually does something. Trileptal, Effexor and Risperdal themselves don’t do squat, but are each transformed into active metabolites that do the real work. Predigested versions of Risperdal and Effexor are now available as Invega and Pristiq. Other meds, like Wellbutrin, do something and are transformed into one or more active metabolites that also do something.
We try to list CYP/UGT/etc in descending order of extent. The first one that does the most of the work, the last one does the least7. That’s how it is on most PI sheets these days.* Topamax is a weak inhibitor of CYP2C19.
- Topamax is a weak inducer of CYP3A4.
- The effect on both enzymes is dose-dependent, and it usually requires a dosage of 400mg a day or higher before Topamax has any effect on drugs metabolized by those enzymes. Keep in mind:
- Topamax is sometimes prescribed at 600–800mg a day for cases of severe epilepsy
- At that dosage it is no longer a weak inhibitor of CYP2C19 or inducer of CYP3A4
- Lots of crazy meds are metabolized by either or both of CYP2C19 & CYP3A4
- Lots of people have epilepsy in addition to other brain cooties
Most drug-whatever interactions involve elimination metabolism. With crazy meds the best known example is Lamictal and Depakote or whichever flavor of carbamazepine you’re taking. If you’re taking Depakote you need to take less Lamictal less often. With Tegretol or Equetro you need to take more Lamictal more often. That’s not always the case. Interactions can affect transformation, which is why Paxil + tamoxifen = that breast cancer isn’t going anywhere, because tamoxifen itself doesn’t do shit, your liver turns it into endoxifen (and perhaps others), which does all the dirty work. Interactions can also be potentiations, where one drug enhances the effect of another without slowing its clearance. All patient information leaflets and drug-drug interaction checkers have boilerplate potentiation interactions along the lines of, “two or more antidepressants meds can make you extra drowsy/spacey/ready to go on a tri-state multicide spree.” Alcohol potentiates the sedative effects of TCAs, while alcohol and benzodiazepines potentiate each other8. There can be interactions that are beneficial, and those rarely show up in PI sheets or drug-drug interaction checkers.
Don’t mix Topamax with ethinyl estradiol (estrogen)-based birth control.
Don’t take Topamax with carbonic anhydrase inhibitors like acetazolamide and brinzolamide.
Topamax + phenytoin or fosphenytoin = less Topamax and more phenytoin/fosphenytoin.
Topamax can cause lithium levels to change in an unpredictable manor. In it states:
In patients, the pharmacokinetics of lithium were unaffected during treatment with topiramate at doses of 200 mg/day; however, there was an observed increase in systemic exposure of lithium (27% for Cmax and 26% for AUC) the US PI sheet
Add to that these two cases reports, Does topiramate elevate serum lithium levels? and Topiramate can cause lithium toxicity and similar warnings in PI/SPCs from other countries.
However, a study referenced in Antiepileptic Drugs has Topamax lowering lithium levels 11% to 16%. The anecdotal evidence is all over the map. So neither, either, or both could happen to you over the course of time. Lithium is almost entirely processed by your kidneys; Topamax is mostly processed by your kidneys and works them pretty hard, so the odds are you’ll retain more lithium if you take the two of them together.
Taking Topamax and Glucophage (metformin HCl) together may not be a good idea, since they do the exact same thing. Both lower blood sugar by being inhibitors of gluconeogenesis, which is the primary mechanism by which Topamax causes weight loss; and why it was initially investigated as diabetes drug back in the 1970s & 80s.
Topamax’s affect on metformin’s clearance, etc. is minor. You’d get the equivalent of a 25% increase in metformin dosage. Since they do the same thing, but there’s not a lot of information on how well Topamax works in that regard, there’s no way to tell how much of Topamax equal how much metformin.
It’s always a good idea to check for drug-drug interactions yourself. Just because most people in the crazy meds business know about really important interactions (e.g. MAOIs and a lot of stuff, warfarin and everything on the planet) doesn’t mean the person who prescribed your meds told you about them, or the pharmacist has all the meds you take at their fingertips like they’re supposed to. Or they have the time to do their jobs properly when not dealing with complete idiots or playing Angry Farmers on the Faecesbooks.
Bioavailability is what PK is really all about. How well a drug is distributed through your body after you take it is important to understand for two critical aspects of pharmacology: these are the numbers a generic manufacturer has to meet for a drug to be considered as bioequivalent; and they help to explain why the raw numbers you see on the SSRI/SNRI equivalence page don’t seem to add up.
If these data aren’t entered, look them up in the pharmacokinetics section of the PI sheet. These data are usually in the US PI sheets - almost always for newer meds, less often in older ones. You’ll also find a wealth of information at Japan’s Pharmaceuticals and Medical Devices Agency site. These data are now appearing in more and more PI sheets / specifications of product characteristics (SPCs) from other countries.
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- AUC/D was 0.72±0.18 hr/L when given rectally. Relative bioavailability of topiramate administered rectally. That’s where I managed to find AUC data for topiramate.
All sorts of PK info at: GlobalRPh’s PK Tools
One more thing - because you usually piss away 70–100% of the remnants of anything you take10, and almost all crazy meds (and all medications in general, including OTC drugs) affect and/or are affected by your liver, if you have a problem with either or both of your liver and kidneys, most, if not all of this information is going to be very different as far as you’re concerned. We can’t cover everything. The PI sheets usually have as much generic information as possible, but there is still a lot of individual math involved. E.g. the Topamax PI sheet gives lots of hard data about how its rate of clearance changes if you have kidney problems based upon creatine clearance and what sort of dialysis treatment you’re undergoing, written in the language of nephrologists. You could bring the PI sheet to your kidney specialist, point to that section, and the doctor would probably be able to tell you how much you’d need to adjust your dosage by. But in the very next paragraph:
In hepatically impaired subjects, the clearance of topiramate may be decreased; the mechanism underlying the decrease is not well understood [see Dosage and Administration (2.7)].--Topamax Full US Prescribing Information
And that’s it. So if you have liver and/or kidney problems, you’ll need to talk to your pharmacist, your doctors, and, if possible, try to get your doctors to talk to each other about it.
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- Poore, Jerod. “Lamictal (lamotrigine): a review of the literature and consumer experience. Highlighting its use, adverse events, and pharmacology from the educated consumers’ perspective.” Crazymeds www.crazymeds.net (2014).
- Ma, Margaret K., Michael H. Woo, and Howard L. Mcleod. “Genetic basis of drug metabolism.” American Journal of Health System Pharmacy 59.21 (2002): 2061-2069.
- Julien, Robert M. Ph.D, Claire D. Advokat, and Joseph Comaty Primer of Drug Action: A comprehensive guide to the actions, uses, and side effects of psychoactive drugs 12th edition Worth Publishers 2011. ISBN:978–1429233439
- McHugh, Josh “Drug Test Cowboys: The Secret World of Pharmaceutical Trial Subjects” Wired Magazine 15.05 (24 April 2007) Revised 10 May 2007
- Topamax (topiramate) Full US Prescribing Information
- Lynch, Tom, PharmD, and Amy Price, MD “The Effect of Cytochrome P450 Metabolism on Drug Response, Interactions, and Adverse Effects” Am Fam Physician. 2007 Aug 1;76(3):391-396.
- Le, Jennifer, PharmD, MAS, BCPS-ID “Pharmacokinetics” Merck Manual for Healthcare Professionals Last revised May 2014
- Pharmacorama’s Pharmacokinetics section
1 Pharmacokinetics literally means how a drug moves through your body, (from the Greek pharmakon - medicine, and kinetikos - to set in motion) and that is exactly what it's about. From the time you swallow (or however it's delivered) a med until you piss it out (which is how most of the remnants of most meds finally leave).
2 However, the FDA requires only a single, frequently sub-therapeutic dose taken by healthy volunteers - who are mostly, sometimes entirely white males between the ages of 20 and 50 willing to take recently developed drugs because they need the money - to determine pharmacokinetic data, including bioavailability and bioequivalence factors.
3 Based on Julien's calculations from A Primer of Drug Action, the half-life multiplied by five is the generally accepted estimate of how long it takes a med to be eliminated from the blood stream/plasma of someone with a normal metabolism.
4 Or whatever. Metabolization, usually via the liver is just one part of process of how a drug is eliminated from your body. Not all drugs are metabolized, but as it's the best-known way many crazy meds are broken down to make it a lot easier for you to piss or push them out, I'm sticking with the less-than-accurate terminology that I've been ignorantly using for nine of the last 10 years.
5 For crazy meds. I have no idea what the average complete clearance is for other types of medications. For all I know there are drugs that utterly vanish from your system in under five passes, and others that won't let go of your squishy bits for years after you stop taking them.
6 AKA 90% of drug-drug interactions.
7 The last ones listed may not do anything at all, as the data about such things are often conflicting.
8 Or: Alcohol + TCAs = Die like a 1960s trophy wife. Alcohol + Benzos = Die like a rock star.
9 The PK of some meds is affected by the dosage (how much you take, e.g. Cymbalta), or the number of doses (how often you take it, e.g. Lamictal). If it's blank then we don't know. Otherwise:
* No: Dosage / Number of doses doesn't matter
* Dosage Proportional: How much you take affects the PK at a steady rate.
* Dosage Non-liner: The dosage affects the PK, but how it does is complicated, or makes no sense at all.
* Number of Doses: How often you take the med affects the PK.
* Other: There's an effect that isn't like any of the above.
* Unknown: We know that they don't know if there's an effect or not.
10 There are always exceptions. With Geodon you crap out 66% of the inactive stuff it's converted to for elimination, and with Latuda (lurasidone) you crap 80% of its leftovers. So anyone who calls Geodon or Latuda "a shitty drug" is a lot closer to the truth than they may realize.
If you have any questions not answered here, please see the Crazymeds Topamax discussion board. We welcome criticisms of the articles, notifications of bad links, site problems, consumer experiences with medications, etc. I’m not always able to write back. Hence I never answer questions about meds via e-mail that are answered by this or other articles. Especially if they have been repeatedly asked on the forum. That’s why we write these damn things. Questions about which meds are best for your condition should also be asked on the forum; because this is a free site, so the price of admission is making things easier for somebody else searching for the same answer. We don’t deal with children on the forum or in private because after doing this for ten years I don’t have the emotional stamina to deal with kids who have brain cooties. How to contact Crazymeds. — Jerod Poore, CME, Publisher Crazymeds (crazymeds.net)
|Last modified on Monday, 01 June, 2015 at 12:04:52 by JerodPoore||Page Author Jerod Poore||Date created Tuesday, 11 January 2011 at 13:43:23|
|“Topamax (topiramate): a Review for the Educated Consumer.” by Jerod Poore is copyright © 2011 Jerod Poore||Published online 2011/01/11|
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1 While there are plenty of books to help you with hypochondria, for some reason there’s not much in the way of websites. Then again, staying off of the Internet is a large part of curing/managing the disorder.
2 Remember kids, Microsloth operating systems are like TOS Star Trek movies with in that every other one sucks way, way more. With TOS Star Trek movies you don’t want to bother watching the odd-numbered ones. With Microsloth OS you don’t want to buy and install the even-numbered ones. Anyone who remembers ME and Vista knows what I mean.
3 Have I mentioned how open source operating systems for commercial applications is one of the dumbest ideas in the history of dumb ideas?* I don’t even need my big-ass rant any more. Heartbleed has made my case for me. And that’s just the one that got all the media attention. The very nature of an open source operating system makes security as much of an illusion as anonymity on teh Intergoogles. Before you flip out too much: the domain Crazymeds is hosted on uses a version of SSL that is not affected by the Heartbleed bug. That’s one of the many reasons why I pay a lot of money and keep this site on Lunarpages.
* Yes, I know I’m using open source browsers. I also test the site using the now-defunct IE and Safari browsers. Their popularity - and superiority - killed IE and Safari, so that’s why I rely on the open source browsers. It’s like brand vs. generic meds. Sometimes the generic is better than the brand.