Shorty64 asked
I have taken only one pill of metronidazole this morning at 10 AM. How long will that one pill stay in my system?
At a glance
- It depends on what you mean by “in my system”
- Metronidazole works against bacteria or protozoa for 6-8 hours after it's absorbed.
- In adults with normal liver function, metronidazole will still be in your system for about 40 hours.
Answer
Hi Shorty and thanks for writing to us.
Metronidazole is an antibiotic or antiprotozoal medication that is used to treat many different types of infections.
When you say how long will this “stay in my system”, I can read this two different ways. I could understand it to mean “how long will a single tablet work against a strain of bacteria?” or “how long until the amount of metronidazole in my system is eliminated entirely?” Since I’m not sure which one you are asking about, I’ll answer both!
If you are asking how long will this work against a bacterial infection? How long is its “duration of action”? Metronidazole reaches its peak concentration in your blood about two hours after you take the dose and kills bacteria or protozoa for 6-8 hours. This is why it’s typically dosed to be taken every 6-8 hours.
If you are asking how long until the amount of metronidazole in my system is eliminated entirely? Even though you took the metronidazole earlier in the day, it’s still in your system.
In adults with normal liver function, metronidazole has a half-life of about 6-8 hours. What this means is that 8 hours after your first dose, if you took a single 500mg tablet your body has metabolized about 250mg of the dose and the other 250mg is still active (it’s actually a bit less since it’s not absorbed 100%). After another 8 hours, about 125mg is still active in your body.
We usually consider a drug to be eliminated after 5 half-lives or in this case, about 40 hours, assuming that you clear half the dose in 8 hours. Folks with liver disease tend to metabolize it much slower; it takes them about 18 hours to clear half the dose. A single 500mg would still be in their system for 90 hours or nearly 4 days. This is also assuming that you were prescribed an immediate-release version. An extended-release version does exist, but it’s seldomly prescribed.
I hope this helps and feel free to write to us again in the future!
References
- Metronidazole Prescribing information, Pfizer
- Metronidazole Pharmacokinetics in Patients with Hepatic Encephalopathy, Scandinavian Journal of Gastroenterology
