Cephadex 500 mg (Cephalexin)

Cephadex 500mg (Cephalexin): Complete Medical Guide — Uses, Dosage, Side Effects & Precautions

Regulatory Note: Cephadex 500mg contains Cephalexin (Cefalexin) 500mg, a first-generation cephalosporin antibiotic, manufactured by Cipla Ltd., India. The originator branded product, Keflex, has been FDA-approved for decades. Cephadex is a generic formulation containing the identical active molecule and is a prescription-only medication. 

Cephadex 500mg is a prescription antibiotic capsule containing Cephalexin 500mg, a beta-lactam antibiotic in the cephalosporin class, structurally related to but distinct from penicillins. It is prescribed for bacterial infections affecting the respiratory tract, urinary tract, skin and soft tissues, ear, throat, and bone. 

A clinically significant interaction absent from all three reviewed source documents: Cephalexin has a well-documented, quantified interaction with metformin, the most widely prescribed medication for type 2 diabetes. A published pharmacokinetic study found that a single 500mg dose of cephalexin increased metformin’s peak plasma concentration by 34% and overall exposure (AUC) by 24%, while reducing metformin’s renal clearance to just 14% of baseline. This finding is now incorporated directly into the FDA-approved cephalexin prescribing label, which specifically advises careful patient monitoring and dose adjustment of metformin in patients taking both drugs together. Given how common metformin use is, this is a genuinely important gap in the source documents’ interaction lists. 

What is Cephadex 500mg? 

Cephadex 500mg is a capsule formulation of Cephalexin, manufactured by Cipla Ltd., India. Cephalexin is effective primarily against gram-positive organisms (such as Staphylococcus and Streptococcus species), with more limited gram-negative coverage compared to later-generation cephalosporins — an important spectrum distinction the source documents do not clarify, which is why cephalexin is typically reserved for specific infection types rather than used as a truly broad-spectrum first choice. 

Cephalexin is excreted almost entirely unchanged in the urine — over 90% of the drug is cleared this way within 8 hours, via both glomerular filtration and active tubular secretion. This near-total renal dependence for elimination is precisely why dose adjustment is required for patients with reduced kidney function (see Dosage table below), and why it interacts with other drugs that also rely on renal tubular secretion, such as metformin and probenecid. 

How Does Cephadex 500mg Work? 

Mechanism of Action 

Like other beta-lactam antibiotics, cephalexin is bactericidal, working by inhibiting bacterial cell wall synthesis. It binds to penicillin-binding proteins (PBPs) located on the bacterial cell membrane, blocking the final cross-linking (transpeptidation) step in peptidoglycan cell wall assembly. Without a properly constructed cell wall, bacteria cannot withstand internal osmotic pressure and lyse (rupture), resulting in bacterial death. This description across the three source documents is broadly accurate but generic — describing “cell wall synthesis” without naming the specific molecular target (PBPs). 

Uses of Cephadex 500mg 

Cephadex 500mg is prescribed for a range of bacterial infections, correctly identified across the source documents: 

• Urinary tract infections (UTIs): cystitis and other uncomplicated lower UTIs 
• Respiratory tract infections: bronchitis and certain pneumonias caused by susceptible organisms
• Ear, nose, and throat infections: otitis media, pharyngitis, tonsillitis 
• Skin and soft tissue infections: cellulitis, boils, wound infections 
• Bone and joint infections: including osteomyelitis, correctly noted by cheap.docx — these typically require the higher end of the dosing range and often longer treatment courses given the need for adequate bone tissue penetration 
• Dental infections 

365.docx correctly notes that severe infections may require parenteral (injectable) treatment rather than oral cephalexin — an important clinical point, since oral cephalexin is generally reserved for mild to moderate infections where adequate oral absorption and tissue penetration can be reasonably expected to clear the infection. 

Dosage and Administration of Cephadex 500mg 

The source documents give reasonably consistent general guidance (every 6–12 hours), but with limited indication-specific detail. A more complete picture: 

 

Indication / Scenario	Typical Regimen
Skin and soft tissue infections	250–500 mg every 6 hours, OR 500 mg every 12 hours
Streptococcal pharyngitis/tonsillitis	500 mg every 12 hours, for 10 days
Uncomplicated cystitis / UTI	500 mg every 12 hours
Otitis media (with adequate weight-based dosing)	Higher doses in divided regimens, weight-based in children
Bone and joint infections (e.g., osteomyelitis)	500 mg every 6 hours (higher end of range, given tissue penetration needs)
Severe infections generally	Higher-end dosing (e.g., 500mg q6h) used for severe/deep-seated infections
Renal impairment (CrCl 30–59 mL/min)	500 mg every 8–12 hours
Renal impairment (CrCl 15–29 mL/min)	250 mg every 8–12 hours
Renal impairment (CrCl 5–14 mL/min)	250 mg every 24 hours

 How to Take Cephadex 500mg 

• Swallow the capsule whole with a glass of water. Do not crush or chew it 
• May be taken with or without food; taking with food can reduce stomach upset if it occurs 
• Space doses evenly throughout the day (e.g., every 6, 8, or 12 hours as prescribed) to maintain consistent drug levels 
• Complete the full prescribed course even if symptoms improve early 

Missed Dose 

Take the missed dose as soon as remembered; skip it if nearly time for the next dose. Do not double the dose. 

Overdose 

365.docx correctly notes that overdose symptoms may include nausea, vomiting, diarrhoea, or blood in the urine. Seek immediate medical attention if an overdose is suspected. 

Seizure Risk in Renal Impairment — An FDA-Labeled Warning Missing from All Three Sources 

None of the three reviewed source documents mention this specific, FDA-labeled warning. The cephalexin prescribing label states: “Several cephalosporins have been implicated in triggering seizures, particularly in patients with renal impairment when the dosage was not reduced.” This is why dose reduction based on creatinine clearance is not merely a general precaution but a specific measure to prevent this serious neurological adverse effect (see the Dosage table above for renal dose adjustments). If seizures occur during cephalexin treatment, the FDA label specifies that the drug should be discontinued immediately, with anticonvulsant therapy given if clinically indicated. 

Side Effects of Cephadex 500mg 

Common Side Effects 

• Diarrhoea — the most frequently reported adverse reaction in clinical trials 
• Nausea and vomiting 
• Dyspepsia (indigestion) and abdominal pain 
• Headache 
• Dizziness 
• Genital or anal pruritus, genital candidiasis, vaginitis, vaginal discharge — correctly implied by superinfection risk in 365.docx, but not specifically named in any source 

Serious Side Effects — Seek Immediate Medical Attention 

• Seizures: Particularly in patients with renal impairment and inadequate dose reduction — discontinue immediately if this occurs (see dedicated section above). 
• Prolonged prothrombin time: Cephalosporins as a class may be associated with this effect, particularly in patients with renal or hepatic impairment, poor nutritional status, or those on a protracted course of therapy or concurrent anticoagulants. Prothrombin time monitoring is advisable in these at-risk groups — a specific point absent from all three source documents. 
• Transient hepatitis and cholestatic jaundice: Reported with cephalosporins as well as penicillins. Yellowing of the skin or eyes, dark urine, or unusual fatigue warrant prompt evaluation. 
• Severe allergic reactions: Anaphylaxis, facial or throat swelling, difficulty breathing, or severe hives require emergency treatment. 
• Clostridioides difficile-associated diarrhoea (CDAD): Severe, persistent, or bloody diarrhoea occurring during or up to two months after treatment. 

Penicillin Allergy and Cephalosporin Cross-Reactivity — Important Context 

All three source documents advise avoiding cephalexin in patients with penicillin allergy, which is appropriate general caution, but this deserves more nuanced context. Cephalosporins and penicillins share a similar beta-lactam ring structure, which historically led to teaching that cross-reactivity occurred in as many as 10% of penicillin-allergic patients. More recent clinical understanding places the true cross-reactivity rate considerably lower for most modern cephalosporins, particularly those (like cephalexin) with dissimilar side chains to the specific penicillin the patient reacted to. Nonetheless, disclosure of any penicillin allergy history to your prescriber remains essential, as the decision to proceed with a cephalosporin in a penicillin-allergic patient should always be individualised based on the severity and nature of the prior reaction. 

 Drug Interactions 

The source documents list several interacting drugs (zinc, cholera vaccine, warfarin, probenecid, methotrexate, furosemide, streptomycin, gentamicin, metformin in 365.docx only) but with minimal explanation of mechanism. The metformin interaction — quantified above — is the most clinically significant addition here: 

 

Drug / Substance	Clinical Action	Mechanism / Risk
Metformin	Monitor for hypoglycaemia; dose adjustment may be needed	Cephalexin increases metformin Cmax by 34% and AUC by 24%, reducing its renal clearance by 14% — a documented, quantified pharmacokinetic interaction
Probenecid	Coadministration NOT recommended per FDA label	Inhibits renal excretion of cephalexin, increasing and prolonging blood levels
Warfarin / oral anticoagulants	Monitor prothrombin time/INR, especially in at-risk patients	Cephalosporins may be associated with prolonged prothrombin time, particularly in renal/hepatic impairment or malnourished patients
Furosemide (and other loop diuretics)	Monitor kidney function	May potentiate nephrotoxic potential of cephalosporins; exact mechanism unclear but documented since the 1970s with related cephalosporins
Aminoglycosides (e.g., gentamicin)	Monitor kidney function closely	Additive nephrotoxicity risk when combined with cephalosporins
Cholera vaccine (live oral)	Avoid concurrent use; space appropriately	Antibacterial activity may inactivate the live vaccine, reducing its effectiveness

 

 Precautions and Warnings 

Allergy 

Do not take Cephadex 500mg if you have a known hypersensitivity to cephalexin or other cephalosporins. Disclose any history of penicillin allergy, understanding the cross-reactivity context discussed above. 

Seizure History 

Given the seizure risk discussed above, disclose any personal history of seizures to your prescriber, and ensure appropriate dose adjustment if you have kidney impairment. 

Kidney Disease 

Dose reduction is required based on creatinine clearance (see Dosage table above), both for general safety and specifically to reduce seizure risk. 

Liver Disease 

Use with caution; periodic liver function monitoring may be advisable given the rare risk of transient hepatitis or cholestatic jaundice. 

Diabetes — Metformin Users 

If you take metformin, inform your prescriber, as cephalexin can meaningfully raise metformin blood levels (see dedicated section above), warranting closer monitoring for hypoglycaemia and possible metformin dose adjustment. 

Gastrointestinal Conditions 

Disclose any history of colitis or gut inflammation, given the risk of antibiotic-associated diarrhoea and CDAD. 

Pregnancy and Breastfeeding 

Cephalexin is classified as FDA Pregnancy Category B — animal studies have not shown evidence of fertility impairment or fetal harm, though adequate, well-controlled human studies are lacking. It passes into breast milk in small amounts; nursing mothers should watch for diarrhoea in the breastfed infant. 

Driving and Alcohol 

Cephalexin does not typically impair driving ability, but avoid driving if dizziness occurs. Alcohol may increase side effects such as dizziness and nausea. 

Storage Instructions 

• Store below 25–30°C in a closed container, protected from light, moisture, and heat 
• Keep out of reach of children and pets 
• Discard capsules once expired 

Frequently Asked Questions (FAQs) 

Does cephalexin interact with metformin? 

Yes, and this is a genuinely important interaction. A published pharmacokinetic study found cephalexin increased metformin’s peak concentration by 34% and overall exposure by 24%, reducing its renal clearance to just 14% of normal. This finding is included in the FDA-approved prescribing label, which recommends careful monitoring and possible metformin dose adjustment if you take both medications together. 

Can Cephadex 500mg cause seizures? 

This is rare but is a specific FDA-labeled warning, occurring particularly in patients with renal impairment when the dose has not been appropriately reduced. This is why dose adjustment based on kidney function is important, not merely a general precaution. 

If I’m allergic to penicillin, can I take Cephadex 500mg? 

This requires individualised medical judgement. While cephalosporins and penicillins share structural similarities, modern understanding places true cross-reactivity considerably lower than the historically cited 10% figure for many cephalosporins, including cephalexin. Always disclose your allergy history and the nature of any prior reaction to your prescriber before starting this medication. 

What is Cephadex 500mg mainly used for? 

It treats bacterial infections of the urinary tract, skin, throat, ear, respiratory tract, and bone (including osteomyelitis). It is not effective against viral infections like the common cold or flu, and is generally reserved for mild to moderate infections — severe infections may require injectable antibiotic treatment instead. 

How long does Cephadex 500mg take to clear an infection? 

This varies by infection type and severity, typically ranging from a 7 to 14-day course depending on the specific indication. Always complete the full prescribed course, even if symptoms improve sooner. 

Medical Disclaimer 

All information in this guide is provided for educational purposes only and does not constitute medical advice, diagnosis, or treatment. Cephadex 500mg is a prescription-only antibiotic and should only be used under the supervision of a licensed healthcare provider. Never self-medicate, share antibiotics with others, or use leftover antibiotics for a different illness. In an emergency, contact local emergency services immediately. Report suspected adverse effects to the FDA at 1-800-FDA-1088 or www.fda.gov/medwatch.