What to Do If You Miss a Dose of Acyclovir for Herpes Zoster
If you miss a dose of acyclovir 800 mg five times daily for herpes zoster, take the missed dose as soon as you remember, but if it's almost time for your next scheduled dose, skip the missed dose and continue with your regular dosing schedule—never double up doses.
Understanding the Standard Regimen
The standard treatment for herpes zoster in immunocompetent adults is acyclovir 800 mg orally five times daily for 7 days 1, 2, 3. This frequent dosing schedule is necessary because:
- Acyclovir has poor oral bioavailability (10-20%) that decreases with increasing doses 2
- The plasma elimination half-life is only 2.5 to 3.3 hours, requiring frequent administration to maintain therapeutic levels 2
- The intracellular half-life of acyclovir triphosphate in VZV-infected cells is approximately 1 hour, much shorter than newer antivirals 4
Practical Approach to Missed Doses
When you realize you've missed a dose:
- If less than 2-3 hours have passed since the scheduled time: Take the missed dose immediately and adjust subsequent doses to maintain approximately equal intervals throughout the day 2
- If you're close to the next scheduled dose (within 1-2 hours): Skip the missed dose entirely and resume your regular schedule—do not take two doses at once 2
- Space remaining doses evenly: If you miss an early dose, redistribute the remaining doses across your waking hours to maintain as consistent drug levels as possible
Critical Timing Considerations
Treatment must be initiated within 72 hours of rash onset for maximum benefit, with greatest efficacy when started within 48 hours 3, 5. Research demonstrates:
- No significant hastening of rash healing occurs in patients who start therapy later than 48 hours after rash onset 5
- Acyclovir significantly reduces time to last new lesion formation, loss of vesicles, and full crusting when given early 5
- Pain reduction during acute disease is significantly greater with timely treatment 6
Common Pitfalls to Avoid
Do not attempt to "catch up" by doubling doses, as this will not improve efficacy and may increase side effects without benefit given acyclovir's dose-dependent decrease in bioavailability 2. The pharmacokinetics show that plasma concentrations increase less than proportionally with dose escalation 2.
Do not extend treatment beyond 7 days in immunocompetent patients unless specifically directed by your physician, as studies show no additional benefit from 21-day courses compared to standard 7-day treatment for preventing postherpetic neuralgia 6.
Special Circumstances
For immunocompromised patients: More aggressive therapy with higher doses or IV administration may be required, and missing doses is more consequential 7. Contact your healthcare provider if you miss doses in this setting.
If you frequently miss doses due to the five-times-daily schedule: Discuss alternative antivirals with your provider, as valacyclovir (1000 mg three times daily) or famciclovir offer simpler dosing with comparable efficacy 4, 8.
Renal Function Considerations
Dosage adjustment is recommended for patients with reduced renal function, and the half-life of acyclovir is dependent on renal function 2. If you have kidney disease and miss doses, consult your provider as your regimen may need individualized adjustment.