How fast does acyclovir start working?
Acyclovir generally starts working shortly after it’s taken, with early symptom improvement often seen within the first day. For many viral infections treated with acyclovir, patients notice less new lesion formation and reduced pain as treatment continues over the next 48–72 hours.
How quickly you should expect results depends on what you’re treating
The timing of improvement varies by condition and whether treatment starts early:
- Cold sores (herpes labialis): Starting acyclovir soon after symptoms begin is linked to faster drying/healing and less pain. Many people see noticeable changes within 1–2 days.
- Genital herpes (herpes simplex): Symptom improvement is often gradual, with many people seeing the most change over 2–3 days after starting.
- Shingles (herpes zoster): Antiviral treatment works best when started early (ideally within 72 hours of rash onset). Pain relief and decreased progression of lesions typically take a few days.
What matters most: when you start vs. the dose schedule
Acyclovir works best when started early in the course of the outbreak. Even if it begins acting quickly, you may not feel full relief immediately because the virus has already triggered inflammation and tissue changes.
When to call a clinician
Seek medical advice if:
- symptoms are worsening after 48–72 hours of treatment,
- there are signs of severe infection (e.g., high fever, rapidly spreading rash),
- you’re immunocompromised, or
- symptoms are in or near the eye.
If you tell me which condition you’re treating (cold sore, genital herpes, shingles, or something else) and whether you’re using tablets or a different form, I can give a more specific expected timeline.