NutriStack
Cephalexin and Vitamin K2, a caution.
Broad-spectrum antibiotics including cephalosporins suppress vitamin K-producing gut bacteria, which can lower endogenous vitamin K2 (menaquinone) levels during prolonged courses. In patients with marginal vitamin K intake, malnutrition, or coagulopathy risk, this can increase bleeding tendency. Routine supplementation is generally not required for short courses in well-nourished adults.
One pair, every claim cited. The two substances, the type, the mechanism, the recommendation, and the primary literature.
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At a glance
- Substances
- Cephalexin and Vitamin K2
- Pair type
- Caution
- Evidence (highest tier)
- Moderate
- Source citations
- 2 sources
- Stack Score effect
- −5 to your Stack Score (per scored caution row).
- Scope
- Supplement × Prescription
- Last verified
- May 30, 2026
Caution · Moderate evidence
Caution
What is happening. Broad-spectrum antibiotics including cephalosporins suppress vitamin K-producing gut bacteria, which can lower endogenous vitamin K2 (menaquinone) levels during prolonged courses. In patients with marginal vitamin K intake, malnutrition, or coagulopathy risk, this can increase bleeding tendency. Routine supplementation is generally not required for short courses in well-nourished adults.
Mechanism. Cephalosporins disrupt the colonic anaerobic flora responsible for synthesizing vitamin K2 (menaquinones), gradually depleting hepatic menaquinone stores and reducing gamma-carboxylation of clotting factors II, VII, IX, and X.
Recommendation. For short cephalexin courses in healthy adults, no special action is needed. For prolonged therapy, malnourished patients, or those at bleeding risk, consider monitoring coagulation parameters and discussing vitamin K2 supplementation with your clinician.
Sources (2)
- Shirakawa H, Komai M, Kimura S. Antibiotic-induced vitamin K deficiency and the role of the presence of intestinal flora. Int J Vitam Nutr Res. 1990;60(3):245-51. PMID 2276882
- Conly J, Stein K. Reduction of vitamin K2 concentrations in human liver associated with the use of broad spectrum antimicrobials. Clin Invest Med. 1994;17(6):531-539. PMID 7895417
Stack Score
How this pair moves the number.
Effect on the composite score
If both Cephalexin and Vitamin K2 are in the same stack, this pair applies −5 to your Stack Score (per scored caution row).
The full algorithm, the clamping rules, and four worked stacks are documented at /methodology/stack-score.
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