[4] Cautions include known sensitivity to beta-lactam antibacterials, such as penicillins (Cefaclor should be avoided if there is a history of immediate hypersensitivity reaction); renal impairment (no dose adjustment required, although manufacturer advises caution); pregnancy and breast-feeding (but appropriate to use); false positive urinary glucose (if tested for reducing substances) and false positive Coombs test.

The frequency and severity of serum sickness-like reactions in children has led researchers to question its role in pediatric illness.

In the UK, The Committee on the Safety of Medicines (CSM) has warned that the risk of diarrhea and rarely antibiotic-associated colitis are more likely with higher doses.

[citation needed] Cefaclor is passed into the breast milk in small quantities, but is generally accepted to be safe to take during breastfeeding.

[10] Cephalosporins possibly enhance the anticoagulant effect of coumarins (e.g. Warfarin) - change in patient's clinical condition, particularly associated with liver disease, intercurrent illness, or drug administration, necessitates more frequent testing of INR, and dose adjustment as necessary.

[citation needed] Excretion of cephalosporins is reduced by probenecid (resulting in increased concentrations of drug in the blood plasma).