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Cephalexin is currently the 102nd most commonly prescribed drug in the United States. In 2017 alone, there were 7,240,287 prescriptions. Considering how popular this drug is right now, it pays to know a little bit more about this medication, including how long cephalexin stays in your system.

Mean effective half-life: 1-2 hours 

The U.S. National Library of Medicine and StatPearls state the following information regarding cephalexin half-life:

Mean effective half-life: 1-2 hours 

Time to steady-state: it may take up to 48 hours for cephalexin to start improving a person’s infection-related symptoms

Cephalexin has a nearly 100 percent bioavailability. The body rapidly absorbs the drug before eventually excreting it in urine via the kidneys.


What is the half-life of cephalexin?

The half-life of cephalexin is about 1 to 2 hours. The half-life may be as long as 16 to 22 hours or more in certain individuals, such as people with impaired kidney function. Taking this drug  with food may also slightly increase its half-life. 

How long does cephalexin stay in your system?

Most (90 percent) of the medication will be out of a person’s system about 6 to 8 hours after taking their last dose. The detection window for this drug is roughly as follows.

  • Urine: Within 8 hours, 90% of the drug leaves the body unchanged via urine; peak urine concentrations after dosing are as follows:
    • 250 mg – 1000 mcg/mL
    • 500 mg – 2000 mcg/mL
    • 1 g – 5000 mcg/mL
  • Serum: Detectable (0.2 mcg/mL) up to 6 hours after last dose

How long does it take for cephalexin to get out of your system?

It may take up to one day (24 hours) for cephalexin to get out of your system. This clearance may take longer if you have reduced kidney function or other health problems.

How long does it take for antibiotics to leave my system?

Most drugs, including antibiotics, take a length of time equivalent to about 4 to 5 half-lives in order to be fully removed from the system. 

Does drinking water help flush out medication?

Since cephalexin is eliminated from the body via the urine, drinking water should help the kidneys excrete the drug effectively. Although, it won’t necessarily shorten the amount of time that this medication is in your system.

Disclaimer: this article does not constitute or replace medical advice. If you have an emergency or a serious medical question, please contact a medical professional or call 911 immediately. To see our full medical disclaimer, visit our Terms of Use page.

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