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Medical Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice.
Herpes Simplex Virus Nucleoside Analog DNA Polymerase Inhibitor [EPC]
Acyclovir is a potent nucleoside analog DNA polymerase inhibitor used to treat infections caused by herpes simplex and varicella-zoster viruses. It works by halting viral DNA replication, effectively managing outbreaks of shingles, chickenpox, and genital herpes.
Name
Acyclovir
Raw Name
ACYCLOVIR SODIUM
Category
Herpes Simplex Virus Nucleoside Analog DNA Polymerase Inhibitor [EPC]
Salt Form
Sodium
Drug Count
6
Variant Count
198
Last Verified
February 17, 2026
RxCUI
1734934, 141859, 197311, 197313, 2263503, 2263505, 197310, 197312, 307730, 1734932, 209977, 825321, 199413, 2734142, 998422, 998426
UNII
927L42J563, X4HES1O11F, WI4X0X7BPJ
About Acyclovir
Acyclovir is a potent nucleoside analog DNA polymerase inhibitor used to treat infections caused by herpes simplex and varicella-zoster viruses. It works by halting viral DNA replication, effectively managing outbreaks of shingles, chickenpox, and genital herpes.
Detailed information about Acyclovir
References used for this content
This page is for informational purposes only and does not replace medical advice. Consult a qualified healthcare professional before using any medication containing Acyclovir.
Acyclovir (also known by its chemical name acycloguanosine) is a synthetic nucleoside analog that serves as a cornerstone in the treatment of viral infections caused by the herpesvirus family. Specifically, it is classified as a Herpes Simplex Virus Nucleoside Analog DNA Polymerase Inhibitor. Since its revolutionary discovery by Gertrude Elion—which earned her a Nobel Prize—and its subsequent FDA approval in 1982, Acyclovir has transformed the clinical management of herpes simplex virus (HSV) and varicella-zoster virus (VZV) infections. It is important to understand that while Acyclovir is highly effective at suppressing viral replication, it is not a cure for herpes infections. The virus remains dormant in the nerve ganglia, and Acyclovir works to reduce the severity, duration, and frequency of symptomatic outbreaks.
Acyclovir belongs to a class of drugs called antivirals. It is uniquely engineered to be a 'prodrug' in a sense—it remains largely inactive in the body until it encounters a cell infected by the herpes virus. This selectivity is what makes Acyclovir a gold standard in safety, as it primarily targets viral machinery while leaving healthy human cells relatively unaffected. Healthcare providers typically prescribe Acyclovir for the treatment of genital herpes, cold sores (herpes labialis), shingles (herpes zoster), and chickenpox (varicella). In more severe clinical settings, such as herpes simplex encephalitis or neonatal herpes, the intravenous (sodium salt) form is utilized for rapid systemic intervention.
The mechanism of action of Acyclovir is a masterclass in targeted molecular pharmacology. At the molecular level, Acyclovir mimics deoxyguanosine, one of the building blocks of DNA. The process begins when Acyclovir enters a virus-infected cell. The virus produces an enzyme called thymidine kinase (TK). This viral enzyme is significantly more efficient at activating Acyclovir than any human enzyme. The viral TK converts Acyclovir into acyclovir monophosphate. Subsequently, host cell enzymes further phosphorylate this into acyclovir triphosphate, the active form of the drug.
Once in its triphosphate form, Acyclovir acts as a 'suicide substrate' for the viral DNA polymerase. It competes with the natural deoxyguanosine triphosphate for a spot in the growing viral DNA chain. Because Acyclovir lacks a 3' hydroxyl group—a necessary component for the next 'link' in the DNA chain—once it is incorporated, the DNA chain is terminated. This is known as 'obligate chain termination.' Without the ability to replicate its DNA, the virus cannot produce new virions, effectively halting the spread of the infection within the host. This highly specific mechanism ensures that the drug's inhibitory activity is concentrated where it is needed most, minimizing toxicity to the patient.
Understanding how the body processes Acyclovir is essential for optimizing therapeutic outcomes. The pharmacokinetics of Acyclovir vary significantly depending on the route of administration.
Acyclovir is FDA-approved for several specific indications involving the herpesvirus family:
Acyclovir is available in a diverse array of formulations to suit different clinical needs:
> Important: Only your healthcare provider can determine if Acyclovir is right for your specific condition. The choice of formulation and dosage depends on the severity of the infection and the patient's overall health status.
Dosage for Acyclovir is highly dependent on the condition being treated and the patient's immune status. Healthcare providers typically follow these standard guidelines:
Acyclovir is approved for use in children, but dosing is usually calculated based on body weight or surface area:
Since Acyclovir is primarily cleared by the kidneys, dose adjustments are mandatory for patients with decreased kidney function. If the creatinine clearance (a measure of kidney health) falls below 25 mL/min, the frequency of dosing is typically reduced. For example, in shingles patients with severe renal failure, the dose may be reduced to 800 mg every 12 hours instead of every 4 hours. Failure to adjust the dose can lead to drug accumulation and neurotoxicity.
No specific dose adjustments are usually required for patients with liver disease, as hepatic metabolism plays a minor role in Acyclovir clearance. However, these patients should still be monitored closely.
Older adults are more likely to have reduced renal function and are at a higher risk for side effects like confusion or agitation. Healthcare providers often start at the lower end of the dosing range and emphasize the importance of hydration.
To ensure the medication works effectively, follow these specific instructions:
If you miss a dose of Acyclovir, take it as soon as you remember. However, if it is almost time for your next scheduled dose, skip the missed dose and continue with your regular schedule. Do not 'double up' or take extra medicine to make up for a missed dose, as this increases the risk of side effects.
An overdose of Acyclovir can lead to kidney failure and neurological symptoms. Signs of overdose include:
In the event of a suspected overdose, contact your local poison control center or seek emergency medical attention immediately. Hemodialysis may be required in severe cases to remove the drug from the bloodstream.
> Important: Follow your healthcare provider's dosing instructions exactly. Do not adjust your dose or stop the medication without first consulting a medical professional.
While Acyclovir is generally well-tolerated, some patients may experience mild side effects as their body adjusts to the medication. These common effects include:
> Warning: Stop taking Acyclovir and call your doctor immediately if you experience any of the following serious symptoms. These may indicate a severe reaction or organ toxicity.
For patients on long-term suppressive therapy (taking Acyclovir daily for a year or more), the primary concern is the potential impact on renal function. Clinical studies have shown that long-term use is generally safe for patients with healthy kidneys, provided they remain hydrated. However, periodic monitoring of kidney function (serum creatinine) is recommended. There is no evidence that Acyclovir causes significant cumulative toxicity in other organ systems, but patients should remain vigilant for any new or changing symptoms over time.
Currently, there are no FDA black box warnings for Acyclovir. It has a long-standing safety profile when used according to standard prescribing guidelines. However, the lack of a black box warning does not mean the drug is without risk; the potential for kidney and neurological toxicity remains the most significant clinical concern.
Report any unusual symptoms to your healthcare provider. If you suspect you are having a serious reaction, do not wait for your next appointment—seek urgent care.
Acyclovir is a powerful antiviral, but it must be used with caution in certain populations. The most critical safety factor is maintaining adequate hydration. Because the drug is excreted by the kidneys, it can become highly concentrated in the renal tubules, leading to the formation of crystals that can cause acute kidney injury. Patients should aim to drink at least 8 to 10 glasses of water daily unless otherwise directed by their doctor.
Another key safety point is the 'window of opportunity' for treatment. For conditions like shingles or chickenpox, Acyclovir is most effective when started as soon as possible—ideally within 24 to 72 hours of the first symptom. Delaying treatment may reduce the drug's ability to shorten the duration of the illness or prevent complications.
No FDA black box warnings for Acyclovir. Unlike some other antivirals (such as Ganciclovir), Acyclovir does not carry a warning for severe bone marrow suppression or carcinogenicity in its standard labeling.
If you are taking Acyclovir for a short course (e.g., 5-10 days), intensive monitoring is usually not necessary. However, for those on high-dose IV therapy or long-term suppressive therapy, the following may be required:
Acyclovir can cause dizziness, drowsiness, or confusion in some people, particularly the elderly. You should observe how the medication affects you before driving a car or operating heavy machinery. If you experience any neurological symptoms, avoid these activities entirely and consult your physician.
There is no direct chemical interaction between Acyclovir and alcohol. However, alcohol can cause dehydration, which significantly increases the risk of Acyclovir-induced kidney damage. Furthermore, alcohol can suppress the immune system, making it harder for your body to fight the viral infection. It is generally advised to limit or avoid alcohol during an active viral outbreak.
Do not stop taking Acyclovir abruptly just because your symptoms have disappeared. Stopping the medication too early can allow the virus to begin replicating again, potentially leading to a 'rebound' outbreak that is more severe. There is no 'withdrawal syndrome' associated with Acyclovir, but finishing the full course is essential for therapeutic success.
> Important: Discuss all your medical conditions, especially kidney disease or immune system disorders, with your healthcare provider before starting Acyclovir.
There are few drugs that are strictly 'contraindicated' (never to be used) with Acyclovir, but the use of other nephrotoxic drugs (drugs that damage the kidneys) should be avoided whenever possible. Combining Acyclovir with medications like Amphotericin B (an antifungal) or certain high-dose Aminoglycoside antibiotics (like Gentamicin) can lead to additive kidney damage. If these must be used together, extreme caution and daily monitoring of kidney function are required.
Acyclovir does not have major interactions with specific foods. Unlike some other medications, it is not affected by grapefruit juice or dairy products. However, as previously mentioned, the most important 'food-related' advice is to maintain high water intake. A high-protein diet (which increases the workload on the kidneys) should be balanced with plenty of fluids while on this medication.
Acyclovir generally does not interfere with common laboratory tests, such as blood sugar or cholesterol panels. However, it may cause a false elevation in serum creatinine if the method used for testing is sensitive to the drug's presence. Always inform the lab technician and your doctor that you are taking an antiviral medication.
For each major interaction, the primary mechanism is usually competition for renal tubular secretion or additive nephrotoxicity. The clinical consequence is typically an increased risk of side effects like confusion or kidney strain. Management usually involves either adjusting the dose or increasing fluid intake and monitoring blood work more frequently.
> Important: Tell your doctor about ALL medications, supplements, and herbal products you are taking, including over-the-counter pain relievers.
An absolute contraindication is a condition where the risk of using the drug clearly outweighs any possible benefit. For Acyclovir, these include:
These are conditions where Acyclovir can be used, but only with extreme caution and under close medical supervision:
There is a high degree of cross-sensitivity between drugs in the nucleoside analog class. If you have had a reaction to Famciclovir or Ganciclovir, you should inform your doctor, as your immune system may react similarly to Acyclovir. While they are chemically distinct, their structural similarities can sometimes trigger similar allergic pathways.
> Important: Your healthcare provider will evaluate your complete medical history, including any past drug allergies and your current kidney function, before prescribing Acyclovir to ensure it is the safest option for you.
Acyclovir is classified as Pregnancy Category B. This means that animal reproduction studies have failed to demonstrate a risk to the fetus, and there are no adequate and well-controlled studies in pregnant women. However, Acyclovir has been used extensively in pregnant women for decades. Data from pregnancy registries have not shown an increase in the number of birth defects compared to the general population.
Healthcare providers often prescribe Acyclovir during the third trimester for women with recurrent genital herpes to prevent an active outbreak at the time of delivery, which could be transmitted to the newborn (neonatal herpes). The risk of transmitting a life-threatening infection to the baby usually outweighs the theoretical risks of the medication. Always discuss the risks and benefits with your obstetrician.
Acyclovir does pass into breast milk. Concentrations in breast milk can be 0.6 to 4.1 times the corresponding plasma levels. This would expose a nursing infant to a small dose of the drug (approximately 0.3 mg/kg/day). While this is significantly less than the dose given to treat an infant, caution is advised. Most experts consider Acyclovir compatible with breastfeeding, but the infant should be monitored for any signs of skin rash or upset stomach.
Acyclovir is FDA-approved for use in children as young as 2 years old for chickenpox and even in newborns for neonatal herpes. In children, the drug's safety profile is similar to that in adults. The most important consideration is weight-based dosing. Growth effects have not been observed with short-term or long-term use in the pediatric population. It is NOT typically approved for the treatment of cold sores in children under 12 in its oral form, though topical use may be permitted.
Patients over the age of 65 require special attention. Clinical trials did not include enough subjects over 65 to determine if they respond differently than younger subjects, but clinical experience suggests a higher rate of neurological side effects. This is almost always due to age-related declines in kidney function. Doctors should calculate the 'Creatinine Clearance' for all elderly patients and adjust the dose downward if necessary. Staying hydrated is particularly difficult for some elderly patients, increasing the risk of kidney stones and crystals.
This is the most critical special population for Acyclovir.
No dosage adjustment is recommended for patients with liver cirrhosis or other hepatic issues. However, because these patients may also have secondary kidney issues (hepatorenal syndrome), renal function should still be monitored.
> Important: Special populations require individualized medical assessment. Never share your medication with others, especially those in these high-risk groups.
Acyclovir is a synthetic purine nucleoside analogue. Its primary target is the DNA polymerase enzyme of the Herpes Simplex Virus (types 1 and 2) and the Varicella-Zoster Virus. The drug's genius lies in its three-step activation process:
The relationship between the concentration of Acyclovir and its antiviral effect is well-established. The 'Inhibitory Concentration' (IC50) varies by virus; HSV-1 is the most sensitive, followed by HSV-2, and then VZV. Because VZV is less sensitive, much higher doses (800 mg vs 200 mg) are required to treat shingles than are required for genital herpes. The drug does not have a significant effect on non-infected human cells, which explains its high therapeutic index.
| Parameter | Value |
|---|---|
| Bioavailability | 15% - 30% (Oral) |
| Protein Binding | 9% - 33% |
| Half-life | 2.5 - 3.3 hours (Normal Renal Function) |
| Tmax | 1.5 - 2.5 hours |
| Metabolism | Minimal (<10% hepatic) |
| Excretion | Renal 62% - 91% (Unchanged) |
Acyclovir is the prototype of the Nucleoside Analog Antivirals. Related medications include Valacyclovir (a prodrug of Acyclovir with better absorption), Famciclovir, and Penciclovir. Within the broader therapeutic area, it is categorized as an anti-infective agent specifically targeting the herpesvirus family DNA synthesis process.
Medications containing this ingredient
Common questions about Acyclovir
Acyclovir is primarily used to treat infections caused by the herpesvirus family, including genital herpes, cold sores, shingles, and chickenpox. It works by preventing the virus from replicating its DNA, which helps the body's immune system control the infection more effectively. While it is highly effective at reducing the duration and pain of an outbreak, it is important to note that Acyclovir is not a cure for herpes. The virus remains in the body even after the sores heal, and the medication is used to manage symptoms or prevent future recurrences. Your healthcare provider will determine the appropriate dose based on which specific virus is being treated and your overall health.
Most people who take Acyclovir experience very few side effects, but the most common ones include nausea, vomiting, diarrhea, and headache. These symptoms are usually mild and often go away as your body becomes accustomed to the medication. Some people may also feel a general sense of tiredness or 'malaise' during treatment. Taking the medication with food can often help minimize stomach-related issues. If these side effects become severe or persistent, you should contact your healthcare provider for guidance. Rarely, more serious side effects involving the kidneys or nervous system can occur, particularly if the patient is not drinking enough water.
There is no known direct drug interaction between Acyclovir and alcohol, meaning alcohol does not stop the medication from working. However, drinking alcohol can cause dehydration, which is a significant risk factor for Acyclovir-induced kidney damage. Acyclovir requires the kidneys to be well-flushed with water to prevent crystals from forming in the renal tubules. Additionally, alcohol can weaken your immune system, potentially making it harder for your body to recover from the viral outbreak. For these reasons, healthcare providers generally recommend avoiding or strictly limiting alcohol consumption while being treated for a viral infection.
Acyclovir is generally considered safe during pregnancy and is classified as Category B by the FDA. Extensive data from pregnancy registries have shown that women who took Acyclovir during their first trimester did not have a higher risk of birth defects compared to the general population. In fact, doctors often prescribe Acyclovir near the end of pregnancy for women with genital herpes to prevent an active outbreak during delivery. This is crucial because passing the herpes virus to a newborn can be very dangerous. If you are pregnant or planning to become pregnant, you should discuss the use of Acyclovir with your obstetrician to weigh the benefits against any theoretical risks.
Acyclovir begins working at the molecular level as soon as it enters your bloodstream, but it may take a few days for you to notice a visible improvement in your symptoms. For cold sores or genital herpes, most patients see a reduction in pain and the beginning of sore crusting within 2 to 3 days. For shingles, the pain and rash usually begin to stabilize within 3 to 5 days of starting the medication. The 'speed' of the drug depends heavily on how early you start it; taking it at the very first sign of tingling or itching (the prodrome) yields the best results. Always finish the full course prescribed by your doctor, even if you feel better sooner.
Yes, you can stop taking Acyclovir suddenly without experiencing physical withdrawal symptoms, as it is not an addictive or habit-forming medication. However, it is strongly recommended that you complete the entire course of treatment as prescribed by your healthcare provider. Stopping the medication too early can allow the virus to start replicating again, which may lead to a return of your symptoms or a prolonged healing process. For those on long-term suppressive therapy, stopping the drug will likely result in a return to your previous frequency of outbreaks. Always consult your doctor before deciding to discontinue a prescribed antiviral regimen.
If you miss a dose of Acyclovir, you should take it as soon as you remember. However, if it is almost time for your next scheduled dose, you should skip the missed dose and simply return to your regular dosing schedule. You should never take two doses at once to 'make up' for a missed one, as this can increase the concentration of the drug in your kidneys and raise the risk of side effects. Maintaining a consistent level of the drug in your system is important for suppressing the virus. If you find yourself frequently forgetting doses, consider using a pill organizer or a smartphone alarm to stay on track.
Weight gain is not a recognized or common side effect of Acyclovir. Clinical trials and long-term studies of the drug have not shown any significant link between Acyclovir use and changes in body weight or metabolism. If you experience rapid weight gain or swelling (edema) while taking Acyclovir, this could actually be a sign of a serious problem with your kidneys, which are responsible for fluid balance in the body. In such cases, you should contact your doctor immediately. Otherwise, any changes in weight during a viral illness are more likely related to changes in diet, activity levels, or the body's response to the infection itself.
Acyclovir can be taken with many other medications, but there are some important exceptions you should discuss with your doctor. The most significant interactions occur with other drugs that can affect the kidneys, such as certain antibiotics or high doses of NSAIDs like ibuprofen. Additionally, a drug called Probenecid can slow down the removal of Acyclovir from your body, potentially leading to higher levels and more side effects. Always provide your healthcare provider with a complete list of all medications, vitamins, and herbal supplements you are currently taking. They can check for specific interactions and adjust your dosage if necessary to ensure your safety.
Yes, Acyclovir has been available as a generic medication for many years and is typically very affordable. The brand name version, Zovirax, is still available, but the generic versions contain the same active ingredient and are required by the FDA to meet the same standards for safety, strength, and effectiveness. Generic Acyclovir is available in all common forms, including tablets, capsules, oral liquids, and topical creams. Because it is a 'gold standard' treatment that has been off-patent for a long time, it is included on most insurance formularies and is widely accessible at almost all pharmacies. Choosing the generic version is a cost-effective way to receive high-quality antiviral treatment.