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Writer's pictureTeresa Mendoza, PharmD

Med of the Week: Cephalexin



Cephalexin feature with a picture of an open capsule and bacteria

Cephalexin (Keflex), a first-generation cephalosporin antibiotic, contains many precautions and drug interactions with other antibiotics, anticoagulants, supplements, and Metformin. Dentists should be aware of Cephalexin’s dental treatment and prescribing implications:


Generic Name: Cephalexin

Brand Name(s): Keflex

Pronunciation: sef a LEX in


About this drug:

  • Cephalexin is a cephalosporin antibiotic used to treat bacterial infections, such as in the bladder, ear, skin, lungs, bones, and kidneys, in adults and children at least 1 year old.

  • Cephalexin will not cure the common cold, flu, or viruses.


Dentistry-related tidbits:

  • Cephalexin may cause thrombocytopenia.

  • If a patient is on Cephalexin, be aware that Trimethoprim and tetracycline antibiotics (e.g. Doxycycline, Minocycline, Tetracycline) can alter Cephalexin levels, efficacy, and side effects.

  • Before prescribing Cephalexin, check whether the patient is taking anticoagulants or Metformin. Combining Cephalexin with anticoagulants such as Warfarin, Apixaban, Rivaroxaban, and Dabigatran may increase the risk of bleeding including serious bleeding (especially with high doses of Cephalexin and/or long term therapy). Combining Metformin and Cephalexin may increase the risk of lactic acidosis. Make sure to check for potential interactions before prescribing using a tool like Rx Check.

  • Advise the patient to take Cephalexin with food if they have a sensitive stomach to avoid upset stomach.

  • Instruct the patient to call their physician or the dental office if they develop severe bloody diarrhea before taking any over-the-counter medications.

  • Ask if a patient is taking hormonal birth control (e.g. pills, implants). This medication may decrease the efficacy of the hormonal contraceptive, and the patient may become pregnant. Recommend a backup non-hormonal birth control method such as condoms or diaphragm with spermicide.

  • It is always safest to consult the patient's obstetrician before prescribing any medication for a patient who is pregnant or breastfeeding.

  • This medication may be excreted into breast milk. Exercise caution when prescribing this medication to a breastfeeding patient, as this may lead to sensitization of babies.

  • When breastfeeding there is a possibility of the baby getting diarrhea.

  • Advise patients to seek medical help immediately if they notice signs of a serious allergic reaction, including rash, itching, swelling (especially of the face, tongue or throat), severe dizziness, or trouble breathing.


Fun facts:

  • Cephalexin works by binding to penicillin-binding proteins, inhibiting peptidoglycan synthesis, resulting in inhibition of bacterial cell wall synthesis.

  • Cephalexin mainly covers Gram-positive bacteria (Streptococcus, Staphylococcus) and some Gram-negative bacteria (Proteus, E. coli, Klebsiella).

  • There is a higher risk with first-generation cephalosporins to have cross-reactivity with Penicillin allergy. Do not use Cephalexin in patients with type 1 hypersensitivity to Penicillin that experience swelling, angioedema, or anaphylaxis.

  • Advise patients to take any Zinc supplements at least 3 hours after taking Cephalexin to avoid decreased Cephalexin efficacy.


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