Understanding Valacyclovir and Acyclovir
Valacyclovir (brand name Valtrex) and acyclovir (brand name Zovirax) are antiviral medications primarily used to treat infections caused by the herpes simplex virus (HSV) and varicella-zoster virus (VZV) [1.4.1]. These conditions include genital herpes, cold sores, shingles, and chickenpox [1.2.9, 1.4.4]. While they target the same viruses, their pharmacological properties have important differences.
Acyclovir works by stopping the herpes virus from replicating [1.2.2]. It selectively targets virus-infected cells, where it is converted into its active form, acyclovir triphosphate. This active form inhibits the viral DNA polymerase, an enzyme essential for the virus to make copies of its DNA, thereby halting its spread and giving the immune system a chance to control the outbreak [1.5.1, 1.5.2].
Valacyclovir is what is known as a "prodrug" of acyclovir [1.2.3, 1.5.1]. This means that after oral administration, valacyclovir is rapidly and extensively converted into acyclovir and the amino acid L-valine by the liver and intestines [1.5.1, 1.5.4]. The key advantage of this process is significantly higher bioavailability. Valacyclovir achieves an oral bioavailability of about 54%, compared to just 10-20% for oral acyclovir [1.4.5]. This superior absorption allows valacyclovir to be taken less frequently than acyclovir while achieving the same or even higher concentrations of the active drug in the bloodstream [1.2.2, 1.4.2].
Why You Cannot Take Valacyclovir and Acyclovir Together
Taking valacyclovir and acyclovir together is strongly discouraged and considered unsafe [1.2.3]. Since your body converts valacyclovir into acyclovir, taking both medications simultaneously is equivalent to taking a much higher, unsafe dose of acyclovir [1.2.2, 1.2.5]. This significantly increases the concentration of the active drug in your system, elevating the risk of overdose and serious adverse effects [1.2.1].
The primary danger of combining these drugs is an increased risk of kidney and liver toxicity (nephrotoxicity and hepatotoxicity) [1.3.3]. High concentrations of acyclovir can lead to the formation of drug crystals in the kidney tubules, potentially causing acute kidney injury or permanent renal damage [1.3.3, 1.6.3]. Similarly, the liver can become strained, leading to impaired function [1.3.3]. Elderly patients and those with pre-existing kidney disease are particularly vulnerable to these severe side effects [1.3.7, 1.6.3].
Comparison of Valacyclovir and Acyclovir
While both drugs are effective, the choice between them often comes down to dosing convenience, cost, and the specific condition being treated [1.4.2].
| Feature | Valacyclovir (Valtrex) | Acyclovir (Zovirax) |
|---|---|---|
| Mechanism | Prodrug converted to acyclovir in the body [1.5.1]. | Directly active antiviral agent [1.2.2]. |
| Bioavailability | High (approx. 54%) [1.4.5]. | Low (10-20% orally) [1.4.5]. |
| Dosing Frequency | Less frequent, typically 1-3 times per day [1.4.2, 1.4.5]. | More frequent, up to 5 times per day [1.4.1]. |
| Forms Available | Oral tablets [1.4.1]. | Oral tablets, capsules, liquid suspension, topical cream, intravenous injection [1.4.1, 1.4.4]. |
| Common Side Effects | Headache, nausea, stomach pain [1.4.1]. | Nausea, vomiting, diarrhea, general feeling of being unwell [1.4.1, 1.6.4]. |
| Cost | Can be more expensive [1.4.5]. | Often cheaper [1.4.2]. |
Potential Drug Interactions and Side Effects
Both medications share a similar side effect profile because valacyclovir becomes acyclovir in the body. Common side effects include headache, nausea, diarrhea, and dizziness [1.4.2, 1.6.2].
More serious side effects are rare but can include:
- Kidney Damage: Especially in those who are dehydrated or have pre-existing kidney problems. Signs include decreased urination and swelling in the ankles or feet [1.6.4].
- Nervous System Effects: Confusion, hallucinations, agitation, or seizures, particularly in elderly patients [1.6.1, 1.6.3].
- Allergic Reactions: Rash, hives, and difficulty breathing are possible [1.6.2].
Both drugs can interact with other medications that affect the kidneys, such as certain antibiotics, NSAIDs (like ibuprofen), and immunosuppressants (like cyclosporine), increasing the risk of kidney toxicity [1.2.1, 1.3.3]. It's crucial to inform your healthcare provider of all medications and supplements you are taking [1.3.3]. Additionally, live attenuated vaccines, such as for chickenpox or shingles, should not be administered while taking these antivirals [1.2.5, 1.2.8].
Conclusion
You should not take valacyclovir and acyclovir together. Because valacyclovir is a prodrug that the body metabolizes into acyclovir, combining them leads to an overdose of the active ingredient [1.2.1]. This can result in serious health risks, most notably severe kidney and liver damage [1.3.3]. A healthcare professional will prescribe one or the other based on factors like the specific viral infection, dosing convenience, cost, and your overall health profile. Always follow your doctor's prescription and guidance to ensure safe and effective treatment.
For more in-depth information, you can consult authoritative sources like the FDA drug label for Valtrex.
