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Drug Interactions

Specific Drugs and Laboratory Tests

Pharmacokinetics

Absorption

Bioavailability

Rapidly and completely absorbed from the GI tract. Peak serum concentrations within 1 hour.

Food

Although peak serum concentrations are slightly lower and attained later when administered with food, total amount of drug absorbed is unchanged.

Distribution

Extent

Cephalosporins widely distributed into tissues and fluids.

Distributed into milk.

Plasma Protein Binding

6–15%.

Elimination

Metabolism

Not appreciably metabolized.

Elimination Route

Excreted in urine as unchanged drug by renal tubular secretion and glomerular filtration.

At least 70–90% of a dose eliminated in urine within 8–12 hours in adults with normal renal function.

Half-life

Adults with normal renal function: 0.5–1.2 hours.

Children: about 5 hours in neonates and 2.5 hours in children 3–12 months of age.

Special Populations

Decreased clearance and increased half-life in patients with renal impairment. Half-life is 7.7–13.9 in adults with Cl_cr <13.5 mL/minute.

Stability

Storage

Oral

Capsules

25°C (may be exposed to 15–30°C).

For Suspension

15–30°C in tight, light-resistant container. After reconstitution, refrigerate in a tight container for up to 14 days.

Actions

• First generation cephalosporin with a limited spectrum of activity compared with second and third generation cephalosporins.
• Usually bactericidal.
• Like other β-lactam antibiotics, antibacterial activity results from inhibition of bacterial cell wall synthesis.
• In vitro spectrum of activity includes some gram-positive aerobic bacteria and some gram-negative aerobic bacteria. Inactive against anaerobic bacteria, fungi, and viruses.
• Gram-positive aerobes: active in vitro and in clinical infections against gram-positive aerobic bacteria including Staphylococcus aureus and S. epidermidis (including penicillinase-producing strains), Streptococcus pyogenes (group A β-hemolytic streptococci), and S. pneumoniae. Oxacillin-resistant (methicillin-resistant) staphylococci and most enterococci are resistant.
• Gram-negative aerobes: active in vitro and in clinical infections against some gram-negative aerobic bacteria including Haemophilus influenzae, Moraxella catarrhalis, Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis. Inactive against Acinetobacter, Citrobacter, Enterobacter, Listeria monocytogenes, Morganella morganii, Providencia, Pseudomonas, and Serratia.

Advice to Patients

• Advise patients that antibacterials (including cephalexin) should only be used to treat bacterial infections and not used to treat viral infections (e.g., the common cold).
• Importance of completing full course of therapy, even if feeling better after a few days.
• Advise patients that skipping doses or not completing the full course of therapy may decrease effectiveness and increase the likelihood that bacteria will develop resistance and will not be treatable with cephalexin or other antibacterials in the future.
• Advise patients that diarrhea is a common problem caused by anti-infectives and usually ends when the drug is discontinued. Importance of contacting a clinician if watery and bloody stools (with or without stomach cramps and fever) occur during or as late as 2 months or longer after the last dose.
• Importance of discontinuing therapy and informing clinician if an allergic reaction occurs.
• Importance of informing clinicians of existing or contemplated concomitant therapy, including prescription and OTC drugs as well as any concomitant illnesses.
• Importance of women informing clinicians if they are or plan to become pregnant or plan to breast-feed.
• Importance of informing patients of other important precautionary information. (See Cautions.)