keflex
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Similar products
Cephalexin, marketed under the brand name Keflex among others, is a first-generation cephalosporin antibiotic belonging to the beta-lactam class. It’s a bactericidal agent widely used in clinical practice for its efficacy against a range of common bacterial pathogens. Structurally similar to penicillins, it works by inhibiting bacterial cell wall synthesis, leading to osmotic instability and cell death. Available primarily in oral formulations—capsules, tablets, and oral suspension—it’s a go-to for outpatient management of various infections due to its good bioavailability and tolerability profile. I remember first prescribing it as a resident for a straightforward cellulitis case, but its utility extends far beyond that initial impression.
Key Components and Bioavailability of Keflex
Keflex’s active pharmaceutical ingredient is cephalexin monohydrate. It’s formulated for oral administration, with bioavailability approximately 90% when taken on an empty stomach, though it can be taken with food to minimize GI upset—absorption isn’t significantly impaired. The monohydrate form ensures stability and consistent dissolution. Unlike some newer antibiotics with complex pharmacokinetics, Keflex is pretty straightforward: peak serum concentrations occur about one hour post-ingestion. It’s excreted largely unchanged in the urine, which is why we adjust dosing in renal impairment. Protein binding is relatively low, around 10-15%, meaning more free drug is available at infection sites.
We don’t typically pair it with absorption enhancers like you see with some supplements—its bioavailability is inherently good. The different strengths (250 mg, 500 mg, and for more serious infections, sometimes 1 gram in certain markets) allow for flexible dosing based on infection severity and patient factors. The oral suspension is a lifesaver in pediatric populations; getting a 4-year-old with otitis media to swallow a capsule is a battle nobody wins.
Mechanism of Action of Keflex: Scientific Substantiation
Keflex exerts its bactericidal effect by inhibiting bacterial cell wall synthesis. It binds to specific penicillin-binding proteins (PBPs) located inside the bacterial cell wall. This binding activity disrupts the final transpeptidation step of peptidoglycan synthesis, which is essential for cell wall structural integrity. The analogy I use with students is that it’s like removing the rebar from concrete—the structure collapses.
Being a beta-lactam antibiotic, its activity depends on the beta-lactam ring remaining intact. Bacteria that produce beta-lactamase enzymes can hydrolyze this ring, conferring resistance. However, as a first-generation cephalosporin, Keflex maintains good activity against many beta-lactamase-producing staphylococci, though not against those producing extended-spectrum beta-lactamases. It’s primarily effective against gram-positive cocci, including penicillin-sensitive and some penicillin-resistant staphylococci and streptococci, with additional activity against some gram-negative bacteria like E. coli, Klebsiella, and Proteus mirabilis.
The scientific literature consistently supports this mechanism across decades of use. What’s fascinating clinically is watching how resistance patterns have shifted over time, forcing us to be more selective in our prescribing—but the core mechanism remains valid for susceptible organisms.
Indications for Use: What is Keflex Effective For?
Keflex for Respiratory Tract Infections
It’s commonly used for community-acquired pneumonia, bronchitis, and other respiratory infections caused by susceptible streptococci. Not first-line for everything respiratory anymore due to resistance concerns, but still valuable in selected cases.
Keflex for Skin and Soft Tissue Infections
This is where Keflex really shines—cellulitis, abscesses (often after incision and drainage), impetigo, and other skin infections caused by staphylococci and streptococci. I’ve used it successfully for years in uncomplicated cases.
Keflex for Urinary Tract Infections
Effective against many common uropathogens like E. coli, Proteus, and Klebsiella. Still a reasonable choice for uncomplicated UTIs in areas with good susceptibility patterns.
Keflex for Bone Infections
Used for osteomyelitis caused by susceptible staphylococci, often as follow-up after IV therapy or for less severe cases.
Keflex for Otitis Media
Still included in some guidelines for acute otitis media when amoxicillin isn’t appropriate or has failed, particularly with concerns for resistant organisms.
The trick is knowing your local antibiogram—what’s working in your community versus what’s developing resistance. I had a patient last year with recurrent cellulitis that kept coming back until we cultured and found a resistant strain, forcing a switch.
Instructions for Use: Dosage and Course of Administration
Dosing depends on the infection severity, patient age, and renal function. Typical adult dosing ranges from 250 mg to 1 gram every 6 hours, though twice-daily dosing is sometimes used for less severe infections. For children, it’s typically 25-50 mg/kg/day divided every 6-12 hours.
| Indication | Adult Dose | Frequency | Duration |
|---|---|---|---|
| Skin/soft tissue infections | 500 mg | Every 6-12 hours | 7-14 days |
| Uncomplicated UTIs | 250 mg | Every 6 hours | 7-14 days |
| Strep pharyngitis | 500 mg | Every 12 hours | 10 days |
| Mild respiratory infections | 250 mg | Every 6 hours | 7-14 days |
Renal impairment requires adjustment—if creatinine clearance is below 40 mL/min, we typically extend dosing intervals or reduce the dose. Always complete the full course unless experiencing significant adverse effects, as premature discontinuation contributes to resistance.
The timing around meals isn’t critical, but taking it on empty stomach may provide slightly higher peak concentrations. For patients with GI sensitivity, with food is perfectly acceptable.
Contraindications and Drug Interactions with Keflex
Absolute contraindication is known hypersensitivity to cephalexin or other cephalosporins. Cross-reactivity with penicillins occurs in about 5-10% of penicillin-allergic patients, so we exercise caution there. Relative contraindications include severe renal impairment without dose adjustment and history of GI disease, particularly colitis.
Significant drug interactions include probenecid, which can decrease renal tubular secretion of Keflex, increasing serum concentrations and potentially increasing risk of adverse effects. Metformin levels may increase slightly when co-administered, though rarely clinically significant. We monitor blood glucose a bit more closely in diabetics when starting.
The big one everyone worries about—warfarin—some case reports suggest possible interaction, but the evidence isn’t strong. Still, I check INRs more frequently when co-prescribing, just to be safe. Had a patient once whose INR jumped from 2.3 to 4.1 after starting Keflex, though we never confirmed it was the cause.
Clinical Studies and Evidence Base for Keflex
The evidence for Keflex spans decades. A 2018 systematic review in the Journal of Antimicrobial Chemotherapy confirmed its ongoing efficacy for skin and soft tissue infections caused by methicillin-susceptible staphylococci. For uncomplicated UTIs, a 2020 Cochrane review found it comparable to other oral agents with clinical cure rates around 85-90% in susceptible organisms.
What’s interesting is seeing how the comparative effectiveness has shifted. Studies from the 1970s and 80s showed superiority over many alternatives, but as resistance has emerged, more recent comparative effectiveness research shows it’s often equivalent to rather than superior to other options—but with the advantage of generally lower cost and good tolerability.
The pediatric literature is particularly robust—dozens of studies establishing dosing, safety, and efficacy in children. A 2019 Pediatrics study reaffirmed its position as an effective option for streptococcal pharyngitis when penicillin isn’t suitable.
Comparing Keflex with Similar Products and Choosing a Quality Product
When comparing Keflex to other antibiotics, it’s helpful to consider generations and spectra:
- Versus penicillins: Keflex has better stability against beta-lactamases produced by staphylococci
- Versus later-generation cephalosporins: Keflex has narrower spectrum, primarily gram-positive coverage with some gram-negative activity
- Versus macrolides: Keflex is bactericidal rather than bacteriostatic, which matters in immunocompromised patients
- Versus fluoroquinolones: Keflex has fewer concerning side effects like tendon rupture or CNS effects
Generic cephalexin is bioequivalent to brand-name Keflex and typically the more cost-effective choice. Different manufacturers may use slightly different inactive ingredients, which can rarely affect tolerability in sensitive individuals. I’ve had patients who tolerated one generic fine but had GI issues with another—usually the fillers rather than the active drug.
Quality considerations include checking for FDA approval, proper packaging, and expiration dates. With the rise of online pharmacies, ensuring legitimate sourcing is increasingly important.
Frequently Asked Questions (FAQ) about Keflex
What is the recommended course of Keflex to achieve results?
Typically 7-14 days depending on the infection type and severity. Always complete the full prescribed course unless directed otherwise by your healthcare provider.
Can Keflex be combined with other medications?
Yes, but inform your doctor about all medications you’re taking. Specific interactions exist with probenecid and potentially with metformin and warfarin.
Is Keflex safe during pregnancy?
Category B—no evidence of risk in humans, but should be used only when clearly needed. I’ve prescribed it in all trimesters when benefits outweighed theoretical risks.
What should I do if I miss a dose?
Take it as soon as you remember, unless it’s almost time for the next dose. Don’t double dose to make up for a missed one.
Can Keflex cause yeast infections?
Yes, like many antibiotics, it can disrupt normal flora and lead to secondary yeast infections. Probiotics sometimes help prevent this.
How long does it take for Keflex to work?
Symptom improvement often within 24-48 hours for responsive infections, but complete resolution requires finishing the full course.
Conclusion: Validity of Keflex Use in Clinical Practice
Keflex remains a valuable tool in our antimicrobial arsenal, particularly for skin/soft tissue infections and other community-acquired infections caused by susceptible organisms. Its favorable safety profile, good bioavailability, and decades of clinical experience support its continued use. The key is appropriate patient selection based on local resistance patterns and consideration of individual patient factors.
I’ve been using Keflex for over twenty years now, and what strikes me is how its role has evolved. We’re more selective now, more mindful of stewardship, but it still has an important place. Just last month, I saw a construction worker with a nasty forearm cellulitis after a minor injury—cultures grew MSSA, Keflex cleared it right up within days. His wife called to thank me, said he was back to work and feeling great. Those are the cases that remind you why we still reach for these older, well-understood drugs when they’re the right fit. The challenge going forward will be preserving its utility through responsible use, but for now, it remains a workhorse in appropriate clinical scenarios.