This 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 is4. How it affects the metabolism of other meds5. 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. §2.1 Elimination method CYP/UGT/etc. are listed in descending order of importance/extent/the one that does the most of the work to the one that does the least. As yet undetermined. But whatever does it, Tegretol (carbamazepine) and Dilatin (phenytoin) affect it, as either or both of those drugs speed up the clearance of Topamax by 100%, and both Lamictal and valproates have a mild effect. My money is on a UGT. Regardless, you piss away anywhere from 40–80% of the Topamax you take as unchanged Topamax. (PI sheet: 70%, various studies and dosage-dependent PK account for the range). And dialysis will just suck it right out of you. Only lithium makes your kidneys work any harder to process a crazy med. §2.2 Transformation method N/A §2.3 Active metabolites None §2.4 Inhibits / induces / suppresses 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 One more thing - because you usually piss away 70–100% of the remnants of anything you take, 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:
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