Valtrex vs Acyclovir: Cold Sores, Genital Herpes, Dosing & Safety

Quick Answer: Valtrex (valacyclovir) is converted to acyclovir in the body. The Valtrex label reports acyclovir bioavailability of about 55% after valacyclovir, versus 10% to 20% for oral acyclovir per the acyclovir label, so valacyclovir is taken once or twice daily instead of up to five times. Valtrex is labeled for cold sores, genital herpes (treatment, suppression and reducing transmission), shingles and chickenpox; acyclovir tablets are labeled for genital herpes, shingles and chickenpox. The CDC 2021 STI guidelines list both for genital herpes. Both need dose adjustment for kidney impairment and require a prescription.

Valtrex vs Acyclovir at a Glance

Feature	Valtrex (valacyclovir)	Acyclovir
Form	Prodrug converted to acyclovir	Active drug
Oral bioavailability (labels)	About 55% (as acyclovir)	10%–20%
Cold sores (label)	2 g every 12 hours for 1 day	Not a labeled use for oral tablets
Genital herpes, recurrent (label)	500 mg twice daily for 3 days	200 mg 5 times daily for 5 days
Suppression (label)	500 mg–1 g once daily	400 mg twice daily
Shingles (label)	1 g 3 times daily for 7 days	800 mg 5 times daily for 7–10 days
Reduce transmission to partner	Labeled (500 mg once daily)	Not labeled
Kidney cautions	Dose adjust; acute kidney failure warning	Dose adjust; kidney warnings

Doses above are from the current U.S. Drug Facts or prescribing labels listed in Sources. Always follow the label on the package you buy, because store brands and formulations differ.

How They Work

Acyclovir blocks the DNA polymerase enzyme that herpes viruses (herpes simplex types 1 and 2 and varicella-zoster) need to copy themselves. It does not eliminate the virus, which stays dormant in nerve cells, but it shortens outbreaks and can prevent recurrences. Valacyclovir is acyclovir with an amino acid (valine) attached; the body cleaves it off after absorption. The Valtrex label reports acyclovir bioavailability of 54.5% after a 1-gram dose of valacyclovir, while the acyclovir tablet label lists 10% to 20%. Spruance and colleagues summarize this as a three- to fivefold increase in acyclovir exposure.

Labeled Uses

The Valtrex prescribing information lists, for adults: cold sores (herpes labialis); genital herpes treatment (initial or recurrent episodes); suppression of recurrent genital herpes; reducing transmission of genital herpes; and herpes zoster (shingles). For children: cold sores (12 and older) and chickenpox (2 to under 18). The acyclovir tablet label lists acute treatment of herpes zoster, treatment of initial and management of recurrent genital herpes, and chickenpox.

Dosing Compared

Valtrex label doses for adults with normal kidney function: cold sores 2 grams every 12 hours for 1 day; initial genital herpes 1 gram twice daily for 10 days; recurrent episodes 500 mg twice daily for 3 days; suppression 1 gram once daily (500 mg once daily for people with 9 or fewer recurrences a year); reducing transmission 500 mg once daily; shingles 1 gram three times daily for 7 days.

Acyclovir label doses: initial genital herpes 200 mg every 4 hours, 5 times daily, for 10 days; intermittent therapy for recurrences 200 mg 5 times daily for 5 days; chronic suppression 400 mg twice daily for up to 12 months; shingles 800 mg every 4 hours, 5 times daily, for 7 to 10 days. The CDC’s 2021 STI Treatment Guidelines also list acyclovir 400 mg three times daily for initial genital herpes and several short-course regimens for recurrences.

Fewer daily doses is the main practical advantage of valacyclovir, which can make it easier to finish a course or stay on suppression.

Evidence Highlights

Cold sores: Spruance and colleagues (2003) ran two randomized, placebo-controlled trials of high-dose, short-course valacyclovir started at the first symptoms. In the first study, 2 grams twice daily for 1 day reduced median episode duration by 1.0 day versus placebo, and time to healing and to pain resolution were also shorter. Adverse events were similar to placebo. Starting at the first tingle matters.

Genital herpes suppression: the acyclovir label describes placebo-controlled trials in people with 6 or more recurrences a year in which daily acyclovir for 4 months to 10 years prevented or reduced the frequency or severity of recurrences in more than 95% of patients. Valacyclovir’s label includes a trial showing once-daily suppression reduced transmission to an uninfected partner when combined with safer-sex counseling.

Safety

Kidneys: both drugs are cleared by the kidneys. The Valtrex label warns acute kidney failure can occur, particularly in older adults, people with kidney disease given doses too high for their kidney function, people taking other kidney-toxic drugs and people who are dehydrated. Stay well hydrated and tell your prescriber about kidney problems.
Nervous system effects: agitation, confusion, hallucinations or seizures have been reported, mostly in older adults or people with kidney impairment given excess doses.
TTP/HUS: the Valtrex label notes this blood disorder occurred in people with advanced HIV or transplants receiving 8 grams a day in trials.
Common side effects: headache, nausea and abdominal pain.
Allergy: the acyclovir label contraindicates it in people with significant hypersensitivity to acyclovir or valacyclovir, and vice versa.

What About OTC Options?

For cold sores, docosanol 10% cream (Abreva) is available without a prescription and is applied at the first sign of a sore. Prescription oral antivirals are generally more effective for people with frequent or severe outbreaks; ask a clinician.

How Clinicians Decide

Convenience: valacyclovir’s once- or twice-daily dosing is easier than acyclovir’s up-to-five-times daily dosing.
Cold sores: the 1-day valacyclovir regimen is labeled specifically for this.
Reducing transmission to a partner: valacyclovir has this labeled use.
Liquid formulations or infants: acyclovir is available as a suspension and is the antiviral used in neonatal herpes under specialist care.
Insurance coverage varies; both are available as generics.

Putting It Together: Common Situations

The scenarios below summarize what the labels and guidelines cited on this page say. They are general information, not a personal treatment plan. Your pharmacist can check the choice against your other medicines, and your clinician can tailor it to your health history.

Cold sores in an adult or teen 12 and older: the Valtrex label has a 1-day regimen started at the first symptoms.
Frequent genital herpes recurrences: both labels include daily suppressive therapy; valacyclovir is once daily.
Reduced kidney function or older age: both labels call for dose adjustment, and Valtrex warns about acute kidney failure.

How to Read the Label Before You Buy

Find the active ingredient first. Brand names are often reused across products with different ingredients. The active ingredient and its strength appear at the top of the Drug Facts or Supplement Facts panel.
Check the “Uses” or labeled indication. A product is tested and labeled for specific uses. Using it for something else may not help.
Read the “Do not use” and “Ask a doctor before use” sections. They list conditions and medicines where the product may be unsafe.
Follow the directions for dose and duration. Note the maximum daily amount and how many days you can use it before checking with a clinician.
Check for duplicate ingredients. Combination products can contain the same ingredient as something else you take, which raises the risk of taking too much.
Note the “Stop use and ask a doctor” signs. These are the label’s own warning signs that a product is not working or is causing harm.

Common Mistakes to Avoid

Waiting until a sore is fully formed. Trials started treatment at the earliest symptoms.
Getting dehydrated while taking these drugs. The Valtrex label links dehydration with kidney risk.
Assuming two products with similar names are interchangeable. Compare the active ingredients and strengths instead of the packaging.
Forgetting to tell your clinician. Keep a current list of every prescription, OTC medicine and supplement you take, and bring it to appointments and the pharmacy.

FormulaCompare does not recommend one brand over another. Use the sources below to check the details for the exact product in front of you, since formulas and labels can change.

When to Talk to Your Doctor or Pharmacist

Seek urgent care for a herpes outbreak near the eye, a rash with fever and confusion or severe headache, a widespread rash, or shingles if you are immunocompromised. Contact a clinician about a first genital outbreak, frequent recurrences, decreased urination or confusion while on treatment, and herpes symptoms during pregnancy.

Frequently Asked Questions

Is Valtrex stronger than acyclovir?

Valacyclovir turns into acyclovir, so it is the same active drug. It is absorbed much better, which allows fewer doses per day.

Which is better for cold sores?

Valtrex has a labeled 1-day cold sore regimen (2 grams twice in one day). Oral acyclovir tablets are not labeled for cold sores, though clinicians sometimes use them.

Can Valtrex prevent spreading herpes to a partner?

The Valtrex label includes reducing transmission of genital herpes with 500 mg once daily, along with safer-sex practices. It does not eliminate the risk.

How fast should I start treatment?

As early as possible, ideally at the first tingling or other early symptoms.

Are Valtrex and acyclovir available OTC?

No. Both require a prescription in the US. Docosanol cream for cold sores is OTC.

Do they cure herpes?

No. They shorten outbreaks and reduce recurrences but do not remove the dormant virus.

Sources

Medical disclaimer: This article is for educational purposes and does not replace medical advice. Consult a licensed healthcare provider before starting any supplement, medication, or treatment — particularly if you are pregnant, breastfeeding, taking other medications, or have a diagnosed medical condition.