Keflex (cephalexin) leaves your system quickly. With an elimination half-life of roughly 1 hour, the drug is effectively cleared from your body within 5 to 7 hours after your last dose. Over 90% of it passes through your kidneys and into your urine unchanged, meaning your body doesn’t need to break it down before getting rid of it.
How the Half-Life Works
A drug’s half-life is the time it takes for the amount in your bloodstream to drop by half. Cephalexin’s half-life in adults is approximately 1 hour, though studies have measured it anywhere from 0.7 to 1.1 hours depending on the population. After each hour, roughly half of the remaining drug is gone.
Pharmacologists generally consider a drug eliminated after 5 to 6 half-lives, because by that point less than 2% of the original dose remains. For Keflex, that math works out to about 5 to 7 hours after your final dose. This is fast compared to many antibiotics, which is why Keflex needs to be taken 3 to 4 times per day to keep enough of it in your blood to fight infection.
How Your Body Clears Keflex
Keflex is almost entirely a kidney story. Your kidneys filter it out of the blood through two mechanisms: passive filtration and active transport through specialized channels. According to the FDA’s prescribing information, over 90% of the drug is excreted unchanged in urine within 8 hours of a dose. Very little of it is metabolized or broken down by your liver.
The drug also has very low protein binding, only 10% to 15%. This matters because drugs that cling tightly to blood proteins tend to linger longer in the body. Keflex’s low binding means most of it floats freely in your bloodstream, available for your kidneys to filter out right away. This combination of low protein binding and efficient kidney excretion is what makes it one of the faster-clearing antibiotics.
Factors That Slow Elimination
Kidney function is the single biggest variable. Since your kidneys do nearly all the work of removing cephalexin, anything that reduces kidney function will keep the drug in your system longer. People with significant kidney impairment clear cephalexin more slowly, and their doctors typically adjust the dose or spacing to compensate.
Age plays a role through this same mechanism. Newborns and very young infants have immature kidneys, and research shows they clear cephalexin at roughly one-third the rate of older children and adults. In one study, detectable drug levels were still present 8 to 12 hours after a single dose in infants just days old, compared to the 5 to 7 hour window typical for adults. In healthy older children and adults, kidney function is generally similar enough that age alone doesn’t make a meaningful difference.
Keflex and Drug Tests
Cephalexin is not a controlled substance and is not part of standard drug screenings. However, cephalosporins like Keflex have historically been reported to cause false positives on certain urine tests for opiates, particularly older immunoassay-based screens. If you’re taking Keflex and have a urine drug test coming up, the drug itself should be out of your urine within about 8 hours of your last dose, based on the excretion data showing 90% clearance in that window. Letting the testing facility know you’re on an antibiotic can help avoid any confusion.
Why It Leaves Quickly but Keeps Working
You might wonder how a drug that disappears in a few hours can fight an infection over a 7 to 10 day course. Keflex works by interfering with the ability of bacteria to build their cell walls. It doesn’t need to be present at all times to be effective, but it does need to spend enough time above a certain concentration in your blood during each dosing interval. This is why the typical prescription calls for a dose every 6 to 8 hours: each dose pushes blood levels above that threshold, the drug works on the bacteria during that window, and then clears before your next dose brings levels back up. Missing doses or stopping early shortens the total time the drug spends above that critical concentration, which is why completing the full course matters even though each individual dose leaves your body within hours.