The Journey of Cephalexin Through the Body
Cephalexin, a first-generation cephalosporin antibiotic, is widely prescribed for various bacterial infections, from skin and soft tissue infections to those affecting the respiratory and urinary tracts [1.2.1, 1.10.3]. After oral administration, it is rapidly and almost completely absorbed from the gastrointestinal (GI) tract [1.2.3]. Unlike many other drugs, cephalexin is not metabolized or inactivated in the body [1.8.3, 1.8.5]. This means its chemical structure remains intact from absorption to elimination, a key feature of its pharmacokinetic profile [1.8.2].
The Primary Pathway: Renal Excretion
The fundamental answer to the question, 'How is cephalexin eliminated from the body?' lies with the kidneys. The body gets rid of cephalexin almost exclusively through renal excretion [1.4.2, 1.4.4]. Studies show that more than 90% of an administered dose is excreted unchanged in the urine within a period of 8 hours [1.3.1, 1.5.2]. This rapid and efficient clearance makes it particularly effective for treating urinary tract infections (UTIs), as high concentrations of the active drug are delivered directly to the site of infection [1.3.4, 1.3.5].
The process of renal excretion for cephalexin involves a two-part mechanism:
- Glomerular Filtration: As blood passes through the kidneys, the glomeruli act as filters. Small molecules like cephalexin are filtered from the blood into the urine [1.2.3, 1.3.1].
- Tubular Secretion: In addition to filtration, the kidney tubules actively secrete cephalexin from the blood into the urine [1.2.3, 1.3.1]. This active transport process, involving organic anion transporters (OATs), significantly contributes to the drug's rapid clearance [1.4.3, 1.9.4].
Because it does not undergo hepatic metabolism, the liver plays no significant role in its elimination [1.2.1, 1.8.1]. This is an important distinction from many other medications that are broken down by liver enzymes.
Half-Life and Clearance Rate
The elimination half-life of a drug is the time it takes for the concentration of the drug in the body to be reduced by half. In adults with normal kidney function, cephalexin has a very short half-life of approximately 0.5 to 1.2 hours [1.2.3, 1.4.2]. This short duration is why cephalexin typically requires frequent dosing, such as every 6 or 12 hours, to maintain therapeutic levels in the bloodstream needed to fight infection [1.7.2, 1.4.3]. Within about eight hours, more than 90% of a single dose has been cleared from the system of a healthy individual [1.5.1, 1.5.3].
Factors Influencing Cephalexin Elimination
Several factors can alter how quickly cephalexin is removed from the body. The most significant of these is renal function.
- Renal Impairment: Since the kidneys are almost solely responsible for elimination, any impairment in their function dramatically affects clearance [1.6.2]. In patients with chronic kidney disease (CKD), the half-life of cephalexin is prolonged substantially. For example, in adults with a creatinine clearance below 13.5 mL/minute, the half-life can increase to as long as 7.7 to 13.9 hours [1.4.5]. This necessitates dosage adjustments—either reducing the dose or extending the interval between doses—to prevent the drug from accumulating to toxic levels [1.6.3, 1.7.5].
- Age: Geriatric patients are more likely to have age-related decreases in renal function, even if not formally diagnosed with kidney disease [1.3.1]. Therefore, caution is often exercised when prescribing cephalexin to older adults, and renal function monitoring may be recommended [1.2.4, 1.6.1]. Conversely, elimination half-life is also longer in very young children, being about 5 hours in neonates and 2.5 hours in infants aged 3-12 months [1.2.3].
- Drug Interactions: Certain medications can interfere with cephalexin's elimination. The most notable is probenecid, a drug used for gout. Probenecid competitively inhibits the tubular secretion of cephalexin in the kidneys, which blocks its primary excretion pathway [1.6.5, 1.9.4]. This leads to higher and more sustained blood concentrations of the antibiotic [1.9.1, 1.9.3]. Other medications like metformin can also interact, potentially leading to higher concentrations of metformin [1.6.5].
| Feature | Cephalexin | Cefazolin (Injectable Cephalosporin) |
|---|---|---|
| Primary Route | Oral | Intravenous/Intramuscular |
| Metabolism | Not metabolized [1.8.3] | Not significantly metabolized |
| Elimination | >90% renal (unchanged) [1.3.1] | Primarily renal (unchanged) |
| Half-Life (Normal Renal Fx) | 0.5 - 1.2 hours [1.4.5] | 1.2 - 2.2 hours |
| Protein Binding | Low (10-15%) [1.3.4, 1.8.5] | High (approx. 85%) |
| Effect of Probenecid | Delays excretion [1.6.5] | Delays excretion |
Conclusion
In summary, the elimination of cephalexin from the body is a rapid process managed almost entirely by the kidneys. The drug is filtered and secreted into the urine without being metabolized by the liver. Its short half-life necessitates multiple daily doses, and its clearance is highly dependent on the patient's renal function, age, and potential interactions with other drugs like probenecid. This renal-centric pathway makes monitoring kidney function a critical aspect of ensuring the safe and effective use of this common antibiotic. For further reading, you can consult authoritative sources like the National Library of Medicine's DailyMed.
