Cephalexin for Cold Sores: Why It Doesn't Work & What Does

No. Cephalexin (Keflex) is a cephalosporin antibiotic effective against bacteria. Cold sores are caused by herpes simplex virus type 1 (HSV-1)—a virus. Antibiotics have no effect on viruses.

Effective Cold Sore Treatments

Prevention of Recurrences

• Daily suppressive antiviral (valacyclovir 500–1000 mg/day) if >6 outbreaks/year.
• Identify/avoid triggers: UV exposure, stress, illness, trauma, hormonal changes.
• Lip balm with SPF 30+.

When to See a Doctor

• Frequent/severe outbreaks.
• Immunocompromised.
• Lesions near eyes (herpes keratitis – emergency).
• Neonatal exposure risk.

Cephalexin is for bacterial infections (strep throat, skin infections, UTIs). Using it for cold sores promotes antibiotic resistance and delays effective treatment.

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Category	Examples	Timing
Oral Antivirals	Valacyclovir, acyclovir, famciclovir	Best at prodrome (tingle); shortens by 1–2 days
Topical Antivirals	Docosanol (Abreva), penciclovir, acyclovir cream	Apply 5x/day at first sign
Symptomatic	Lidocaine/benzocaine patches, NSAIDs, ice, lysine (weak evidence)	Pain relief only