How long does acyclovir take to start working?
Acyclovir doesn’t work instantly, but it starts acting after it’s absorbed and reaches infected tissues. For most people, the noticeable effects depend on what you’re treating (for example, herpes outbreaks vs. shingles) and how early you start therapy.
In general, if acyclovir is absorbed properly, you should start to see improvement over the next 1–3 days (with earlier benefit when started right at the first symptoms). The drug’s antiviral effect depends on getting enough of the dose into your body.
If I vomit 1 hour after taking it, will the dose still work?
If you vomit about 1 hour after taking acyclovir, it’s possible some of the dose already got absorbed, but you may not have absorbed the full amount. Whether you need another dose depends on how much you vomited and how soon after swallowing it.
A practical rule of thumb used by many clinicians: if you vomit shortly after taking a medicine (often within about 30 minutes to an hour), the dose may not be fully absorbed. After around an hour, partial absorption is more likely, but you can’t be sure.
Because dosing decisions affect safety and effectiveness, you should contact a pharmacist or your prescriber for specific guidance on whether to repeat the dose in your case.
What should you do right now if you vomited after taking acyclovir?
- Check the tablet/liquid label instructions and any directions your prescriber gave.
- Call your pharmacist for advice on “vomited after 1 hour” dosing for your specific acyclovir formulation (tablet vs. suspension) and strength.
- If you’re having severe vomiting, signs of dehydration, or can’t keep medicines down, get medical advice promptly.
Can I take the next dose as scheduled?
Usually, you would take the next scheduled dose if you’re able to keep it down, but repeating the missed/partially absorbed dose is the part that varies case-by-case. That’s why it’s important to get pharmacist guidance before taking an extra dose.
When to seek urgent help
Get urgent care or emergency help if you have:
- Unable to keep fluids down for many hours
- Signs of dehydration (very little urine, dizziness, fainting)
- Severe illness, confusion, or worsening symptoms
- Eye pain or vision changes (especially with herpes-related symptoms)
If you tell me:
1) what condition you’re treating (cold sores, genital herpes, shingles, chickenpox, something else),
2) the dose you took and whether it was pills or liquid, and
3) how soon after swallowing you vomited and whether you saw the pill in the vomit,
I can help you think through what’s most likely and what to ask your pharmacist.