Cefaclor

For medical students2 min readUpdated 2026-10-10

Cefaclor is an oral $\beta$-lactam antibiotic classified as a second-generation cephalosporin. The drug is well absorbed from the gastrointestinal tract and is used to treat bacterial infections that do not involve the central nervous system.

FormulationsCapsules: 0.25 g and 0.5 g

Dosing frequencyOrally 3 times daily

BBB penetrationDoes not cross the blood-brain barrier

ExcretionExcreted via the kidneys in urine

Classification and Antimicrobial Spectrum

Cephalosporins are divided into generations (I through IV), subdivided into parenteral and oral forms. Cefaclor is a second-generation oral cephalosporin (sharing this subgroup with cefuroxime axetil).

To understand its place in clinical practice, recall the evolution of the drug class: while third-generation agents (e.g., cefixime or ceftriaxone) have an expanded gram-negative spectrum and increased $\beta$-lactamase stability, first- and second-generation drugs (including cefaclor) retain higher activity against gram-positive microorganisms. Note that fourth-generation cephalosporins do not have oral formulations.

Pharmacokinetics

Unlike second-generation parenteral agents (cefuroxime, cefoxitin, cefamandole), which are administered intramuscularly or intravenously every 8 hours, cefaclor is designed exclusively for oral administration.

It demonstrates good gastrointestinal absorption. Once absorbed, the drug distributes widely throughout the body and penetrates many tissues. However, there is a critical limitation: cefaclor absolutely does not cross the blood-brain barrier (BBB). This differentiates it from parenteral cephalosporins, which can penetrate the BBB during meningeal inflammation. The drug is eliminated from the body primarily by the kidneys via urinary excretion.

Adverse Effects

Because cefaclor is a $\beta$-lactam antibiotic, it shares typical adverse reactions common to the entire cephalosporin class.

Common adverse effects: Diarrhea, transient elevations in serum transaminases on liver function tests, and various hypersensitivity (allergic) reactions.
Rare adverse effects: Severe blood dyscrasias (agranulocytosis) and renal toxicity (interstitial nephritis).

Administration and Prescription

The drug is available as 0.25 g and 0.5 g capsules. The standard dosing regimen is 0.25 g orally 3 times daily (note that another second-generation oral agent, cefuroxime axetil, is prescribed twice daily). If necessary, the total daily dose of cefaclor can be increased up to a maximum of 4.0 g.

Example of a prescription in Latin: Rp.: Caps. Cefaclori 0.25 D.t.d. N. 20 S. Take 1 capsule orally 3 times daily.

Mnemonic

Cef-A-clor — Actively Absorbed, but Absolutely does not cross into the brain (BBB).

Frequently asked questions

What specific microorganisms are included in the antibacterial spectrum of cefaclor?

Specific species are not detailed in general overviews, but cefaclor has a broad spectrum covering the following groups:

Gram-negative bacteria ($Gr-$) — exhibits higher activity compared to first-generation agents.
Gram-positive bacteria ($Gr+$) — also covered by the antibacterial spectrum of cephalosporins.

As a second-generation agent, this oral antibiotic complements rather than replaces other therapies.

Which bacterial beta-lactamases is cefaclor resistant to?

Exact enzyme types are not specified, but the drug demonstrates general stability against $\beta$-lactamases. Comparative resistance:

Generation	Susceptibility to Enzymes
1st generation	Hydrolyzed by $\beta$-lactamases
2nd generation (cefaclor)	More stable against $\beta$-lactamases

Cefaclor belongs to the second generation of cephalosporins and shows increased resistance to bacterial enzymes compared to its predecessors.

What are the absolute contraindications to prescribing cefaclor?

Absolute contraindications are not specifically listed in basic summaries. However, during pregnancy, cefaclor requires caution as it may increase the risk of congenital anomalies, necessitating restricted use.

What generation is cefaclor, and how is it administered?

It is a second-generation cephalosporin. Unlike parenteral drugs in this group, cefaclor is administered orally.

Can cefaclor be prescribed for bacterial meningitis?

No. The drug does not cross the blood-brain barrier (BBB) and therefore cannot achieve therapeutic concentrations in the cerebrospinal fluid.

How often must cefaclor be taken?

The standard dosing schedule is 3 times daily. This distinguishes it from cefuroxime axetil, which is taken twice daily.

Go deeper

Differences in pharmacokinetics between oral and parenteral second-generation cephalosporins.
Comparative activity of 1st, 2nd, and 3rd generation cephalosporins against gram-positive flora.
Mechanisms of cross-allergenicity between penicillins and cephalosporins.
Interstitial nephritis as a rare complication of antibiotic therapy: pathogenesis and clinical features.
The concept of prodrugs in pharmacology using cefuroxime axetil as an example.