Cephalexin is a widely prescribed first-generation cephalosporin antibiotic used to combat a variety of bacterial infections [1.5.3, 1.5.5]. A key aspect of its effectiveness and ease of use is its method of delivery into the body. The designated route of administration for cephalexin is exclusively oral [1.2.1, 1.2.5].
The Oral Route: The Standard for Cephalexin
Cephalexin is designed to be taken by mouth and is available in several formulations to suit different patient needs [1.2.4]. This choice of administration is based on its excellent pharmacokinetic profile, particularly its stability in stomach acid and its rapid and complete absorption from the gastrointestinal (GI) tract [1.2.2, 1.4.3].
Available Oral Formulations
To accommodate various patients, from adults to children, cephalexin is offered in multiple oral forms [1.3.2, 1.3.3]:
- Capsules: Available in strengths such as 250 mg, 500 mg, and 750 mg, these are intended to be swallowed whole with water [1.3.2, 1.3.5].
- Tablets: Film-coated tablets are also available, typically in 250 mg and 500 mg strengths [1.3.1, 1.3.3].
- Powder for Oral Suspension: This liquid form is ideal for pediatric patients or adults who have difficulty swallowing pills [1.2.4, 1.8.2]. It is reconstituted with water by a pharmacist before dispensing and comes in concentrations like 125 mg/5 mL and 250 mg/5 mL [1.3.2, 1.2.2]. The suspension must be shaken well before each use and stored in a refrigerator [1.8.2, 1.8.4].
Pharmacokinetics: The Journey of Oral Cephalexin
The effectiveness of oral cephalexin is rooted in how it is absorbed, distributed, metabolized, and excreted by the body [1.4.3].
Absorption
After being taken orally, cephalexin is rapidly and almost completely absorbed from the GI tract [1.2.2]. Peak serum concentrations are typically reached within one hour in fasting adults [1.2.2, 1.5.2]. While taking the medication with food may slightly delay the absorption rate, it does not affect the total amount of the drug absorbed [1.2.2].
Distribution, Metabolism, and Excretion
Once absorbed, cephalexin is widely distributed throughout body fluids [1.4.3]. Only about 10% to 15% of the drug binds to plasma proteins, allowing the majority of it to remain active and fight infection [1.4.3]. A significant feature of cephalexin is that it is not metabolized in the body [1.4.2]. Instead, over 90% of the dose is excreted unchanged in the urine, primarily within 8 hours of administration [1.4.1]. This high concentration of active drug in the urinary system makes it particularly effective for treating urinary tract infections (UTIs) [1.4.3, 1.5.2].
Cephalexin vs. Other Common Antibiotics
Cephalexin is often compared to other antibiotics. Understanding their differences helps in selecting the appropriate treatment.
| Feature | Cephalexin | Amoxicillin | Cefazolin |
|---|---|---|---|
| Route | Oral [1.2.5] | Oral [1.10.3] | Intravenous (IV) or Intramuscular (IM) [1.7.2] |
| Class | 1st-Gen Cephalosporin [1.2.1] | Penicillin [1.10.1] | 1st-Gen Cephalosporin [1.7.1] |
| Common Uses | Skin, respiratory, ear, and urinary tract infections [1.5.3] | Respiratory, ear, and urinary tract infections; strep throat [1.10.1, 1.10.2] | Severe infections, surgical prophylaxis [1.7.1, 1.7.2] |
| Dosing Frequency | Typically 2 to 4 times a day [1.5.1] | Typically 2 to 3 times a day [1.10.1] | Typically every 8 hours [1.7.2] |
Common Indications for Oral Cephalexin
Healthcare providers prescribe cephalexin for infections caused by susceptible bacteria [1.2.5]. Common indications include:
- Respiratory Tract Infections: Such as pharyngitis and tonsillitis caused by Streptococcus pneumoniae and Streptococcus pyogenes [1.2.5, 1.5.5].
- Skin and Skin Structure Infections: Caused by Staphylococcus aureus and/or Streptococcus pyogenes [1.2.5, 1.5.5].
- Otitis Media (Middle Ear Infections): Caused by a range of bacteria including S. pneumoniae and H. influenzae [1.2.5, 1.5.2].
- Genitourinary Tract Infections: Including cystitis and acute prostatitis [1.2.5, 1.5.2].
- Bone Infections: Caused by S. aureus and/or P. mirabilis [1.2.5, 1.5.4].
Important Patient Information
For safe and effective treatment, patients should adhere to specific guidelines.
Administration and Precautions
- Take the medication exactly as prescribed and complete the full course, even if symptoms improve early, to prevent the development of antibiotic resistance [1.8.1, 1.11.1].
- Cephalexin can be taken with or without food, but taking it with food may help if stomach upset occurs [1.2.1, 1.8.2].
- Shake the oral suspension well before each dose and use a calibrated measuring device for accuracy [1.8.3].
- Inform your doctor of any allergies, especially to penicillin or other cephalosporin antibiotics, as cross-sensitivity can occur [1.2.5].
- The most common side effects include diarrhea, nausea, vomiting, and indigestion [1.6.3, 1.6.4]. Contact a healthcare provider if severe or watery diarrhea occurs, as it could be a sign of a more serious condition like C. difficile-associated diarrhea [1.2.5, 1.6.5].
Drug Interactions
Cephalexin can interact with other medications. Co-administration with metformin can increase metformin's plasma concentration [1.2.5, 1.9.3]. It may also interact with probenecid, which can inhibit the renal excretion of cephalexin, and warfarin [1.2.5, 1.9.4]. Cephalexin may also decrease the effectiveness of oral contraceptives [1.6.1, 1.9.4].
Conclusion
The sole route of administration for cephalexin is oral, a decision driven by its excellent absorption, stability, and patient convenience. Available as capsules, tablets, and a liquid suspension, it is a versatile and effective first-generation cephalosporin for treating a wide range of common bacterial infections. Its pharmacokinetic profile, particularly its high rate of unchanged renal excretion, makes it a cornerstone therapy for conditions like UTIs. As with any antibiotic, adherence to prescribed instructions is crucial for maximizing efficacy and minimizing risks.
