What is Acyclovir?
Acyclovir, also known by the brand name Zovirax, is an antiviral medication used to treat infections caused by the herpes virus family [1.2.2, 1.8.4]. This includes treating and preventing:
- Genital Herpes (Herpes Simplex Virus 1 and 2) [1.4.2]
- Cold Sores (Herpes Labialis) [1.5.2]
- Shingles (Herpes Zoster) [1.2.2]
- Chickenpox (Varicella-Zoster Virus) [1.2.2]
Acyclovir is available in several forms, including oral tablets, capsules, liquid suspension, and topical creams [1.8.3]. It works by stopping viruses from multiplying in the body, which helps relieve symptoms and allows infections to clear up faster [1.2.2, 1.4.2]. It is important to note that acyclovir manages symptoms but does not cure the viral infection; the virus remains in the body [1.8.1].
How Acyclovir Works
Acyclovir is a nucleoside analog that selectively targets cells infected with a herpes virus [1.4.2]. Inside an infected cell, a viral enzyme called thymidine kinase converts acyclovir into its active form, acyclovir triphosphate [1.4.4]. This active form does two things to stop viral replication:
- It competitively inhibits the viral DNA polymerase, an enzyme crucial for building new viral DNA [1.4.2].
- It gets incorporated into the growing viral DNA chain, causing termination because it lacks the necessary structure to add the next DNA building block [1.4.2, 1.4.6].
This process is highly selective for virus-infected cells, which explains the drug's relatively low toxicity to human cells [1.4.4].
The Most Common Side Effects
The most common side effect of acyclovir can vary slightly depending on the condition being treated and the dosage form. During clinical trials for shingles (herpes zoster), the most frequently reported adverse event was malaise, a general feeling of discomfort, weakness, or being unwell, reported by 11.5% of patients [1.2.3, 1.3.2].
Other very common side effects across different uses include:
- Nausea and Vomiting: These are among the most frequent reactions, affecting between 2% and 10% of users depending on the length of treatment [1.2.4, 1.2.5, 1.3.2].
- Diarrhea: In studies for chickenpox, diarrhea was the most common side effect, seen in 3.2% of patients [1.2.3, 1.3.2].
- Headache: This is another common complaint, especially with long-term use [1.2.3, 1.3.1].
For topical acyclovir cream, common side effects are localized skin reactions like mild pain, burning, stinging, itching, or dry skin [1.6.1].
Serious but Less Common Side Effects
While most side effects are mild, acyclovir can cause serious adverse reactions in rare cases. It is crucial to be aware of these and seek medical attention if they occur.
- Kidney Problems: Acyclovir can cause kidney damage or acute kidney injury, especially with IV administration or in patients who are dehydrated [1.2.1, 1.2.3]. The risk is lower with oral doses but still present. Staying well-hydrated by drinking plenty of water is essential to help prevent this [1.8.5]. Signs of kidney problems include decreased urination, swelling in the legs or ankles, and flank pain [1.2.1, 1.2.6].
- Neurological Effects: Serious central nervous system side effects can occur, particularly in the elderly or those with kidney impairment [1.7.4]. These may include confusion, hallucinations, agitation, tremors, seizures, and in severe cases, encephalopathy or coma [1.3.4, 1.7.1].
- Severe Allergic Reactions: Though rare, a severe allergic reaction (anaphylaxis) is possible. Symptoms include hives, rash, itching, difficulty breathing, and swelling of the face, tongue, or throat [1.7.1, 1.7.2].
- Blood Disorders: In rare instances, particularly in immunocompromised patients, acyclovir has been linked to serious blood conditions like thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS) [1.2.1, 1.7.3].
Comparison with Other Antivirals
Acyclovir is a first-generation antiviral. Newer drugs, valacyclovir (Valtrex) and famciclovir (Famvir), are also used to treat herpes infections. They are 'prodrugs,' meaning they are converted into their active forms after being absorbed by the body [1.5.2].
| Feature | Acyclovir | Valacyclovir | Famciclovir |
|---|---|---|---|
| Active Form | Acyclovir | Acyclovir [1.5.4] | Penciclovir [1.5.2] |
| Bioavailability | Low (10-20%) [1.4.1] | Higher (54-70%) [1.5.5] | High (77%) [1.5.5] |
| Dosing Frequency | 2 to 5 times per day [1.8.3] | Less frequent, often 1-3 times per day [1.5.3] | Less frequent [1.5.3] |
| Common Side Effects | Malaise, nausea, vomiting, diarrhea, headache [1.2.3] | Headache, nausea, abdominal pain [1.5.4] | Headache, nausea, diarrhea [1.5.4] |
Valacyclovir's higher bioavailability means more of the drug enters the bloodstream, allowing for less frequent dosing than acyclovir [1.4.1, 1.5.3]. Famciclovir is also dosed less often and is a prodrug of penciclovir [1.5.2, 1.5.3]. The choice between these medications depends on the specific condition, cost, and patient tolerance.
How to Manage Side Effects
Most common side effects like nausea can be managed by taking the medication with food [1.6.1]. For headaches, resting and staying hydrated can help [1.6.1]. The most critical step to prevent more serious side effects is to drink plenty of fluids throughout the day to support kidney function [1.8.5]. Always follow your healthcare provider's instructions and report any bothersome or severe side effects immediately [1.6.5].
Conclusion
Acyclovir is a well-established and effective antiviral medication. While its most common side effects are generally mild and manageable—primarily malaise, nausea, vomiting, and diarrhea—it carries a risk of more serious effects, especially on the kidneys and nervous system [1.2.3, 1.7.4]. Understanding these potential effects, staying hydrated, and maintaining open communication with a healthcare provider are key to using acyclovir safely and effectively.
For more detailed information, consult the official patient information from a trusted source, such as MedlinePlus. [1.7.1]
