Acyclovir Injection: What It Treats, IV Dosing, and Side Effects
Last updated September 6, 2026.
**Acyclovir injection is an antiviral medicine, a nucleoside analogue, given into a vein to treat serious herpes virus infections.** Herpes viruses copy themselves using an enzyme called DNA polymerase, and acyclovir gets built into the growing viral DNA and stops that copying, which lowers the amount of virus your body has to fight. It does not cure herpes infection or clear the virus from your body, so outbreaks can still return later. The safety point that matters most is your kidneys: acyclovir leaves the body in urine and can crystallize in the kidney tubules, so it must be infused slowly over at least one hour with plenty of fluids, never as a rapid push or a shot into a muscle.
What does it treat?
Acyclovir injection is used when a herpes infection is severe enough that pills are not enough or you cannot keep pills down. Common uses include initial genital herpes that is severe, with widespread painful ulcers, fever, trouble passing urine, or hospitalization; herpes simplex infections of the skin, mouth, or bloodstream in people whose immune system is suppressed by chemotherapy, transplant medicines, advanced HIV, or leukemia; herpes simplex encephalitis, a brain infection with fever, confusion, personality change, headache, or seizures; shingles and chickenpox caused by varicella zoster virus in people who are immunocompromised, including painful blistering rash spreading beyond one skin band; and herpes infection in newborns, which can show as skin blisters, poor feeding, sleepiness, or seizures in the first weeks of life. It is a single-ingredient product, so there are no combination versions, though it is often given alongside pain medicine, IV fluids, and antibiotics while the cause of the illness is being sorted out.
How do you take it?
Acyclovir injection comes as a solution or powder in vials and is diluted and infused into a vein over at least one hour by a nurse or trained home infusion provider, never as an intramuscular, subcutaneous, or eye medicine. Typical adult label doses are 5 mg/kg every 8 hours for 5 days for severe initial genital herpes, 5 mg/kg every 8 hours for 7 days for herpes simplex infections in immunocompromised patients, and 10 mg/kg every 8 hours for 10 days for herpes simplex encephalitis or for shingles and chickenpox in immunocompromised patients, with each dose based on body weight and using ideal body weight if you are obese. The usual adult ceiling is 10 mg/kg every 8 hours, which is 30 mg/kg per day, and doses are spaced further apart when kidney function is reduced: every 12 hours for a creatinine clearance of 25 to 50, every 24 hours for 10 to 25, and half the dose every 24 hours below 10, with a dose given after each dialysis session. Food does not affect it because it goes into a vein, but drinking fluids steadily through the day matters, and your team will check kidney blood tests during treatment. If you are on home infusion and miss a dose, start it as soon as you remember unless your next dose is close, then skip the missed one and go back to schedule without doubling up. Call your doctor before your next dose if you are passing much less urine, your hands or feet swell, the infusion site becomes red, hard, or leaks, or you feel confused, shaky, or unusually sleepy.
Side effects to know
- Common: Many people notice soreness, redness, or a firm tender vein at the IV site, along with nausea, vomiting, headache, or itching, and blood tests may show a temporary rise in kidney numbers.
- Serious: Stop the infusion and get help right away for signs of kidney injury such as passing very little urine, swelling, and rapid weight gain, or for nerve effects such as confusion, hallucinations, agitation, tremor, slurred speech, or seizures, which are more likely with high doses, dehydration, older age, or reduced kidney function. Also get help right away for hives, wheezing, or swelling of the lips, tongue, or throat, and for unexplained bruising, tiny red skin spots, or bleeding, which in people with a suppressed immune system can signal a serious blood and kidney problem called thrombotic thrombocytopenic purpura or hemolytic uremic syndrome.
Who should not take it?
Do not use acyclovir injection if you have had an allergic reaction to acyclovir or valacyclovir. Tell your clinician before starting if you have kidney disease, are dehydrated, are over 65, have had seizures or a neurological condition, or have a suppressed immune system, since all of these raise the chance of kidney and nerve side effects and often change the dose or the spacing between doses. Interactions matter: probenecid raises acyclovir levels, zidovudine can add drowsiness and fatigue, and combining acyclovir with other medicines that stress the kidneys such as aminoglycoside antibiotics, amphotericin B, cyclosporine, tacrolimus, IV contrast dye, or regular NSAIDs like ibuprofen or naproxen increases the risk of kidney injury, so bring a full list of prescriptions, over the counter products, and supplements. Available data from published studies and pregnancy registries have not identified a drug-associated risk of major birth defects, and acyclovir is used in pregnancy when a herpes infection is serious, so this is a discussion to have with your obstetric clinician rather than a reason to refuse treatment. Acyclovir passes into breast milk at levels higher than in your blood, and infant exposure is low with normal feeding, so tell your clinician if you are nursing and avoid feeding from a breast with an active herpes lesion. There is no direct interaction with alcohol, but drinking makes you lose fluid and can make nausea and dizziness worse, so it is better to skip alcohol while you are on IV treatment and keep drinking water instead.
When is it an emergency?
Call 911 in the US if you collapse, have a seizure, cannot be woken, or have trouble breathing, wheezing, or swelling of the face, lips, tongue, or throat during or after an infusion. For a suspected overdose or an accidental extra dose, call Poison Control at 1-800-222-1222 right away, even if you feel fine; too much acyclovir can crystallize in the kidneys and cause sudden kidney failure, and it can cause agitation, confusion, hallucinations, seizures, or coma, which may need IV fluids and sometimes dialysis. Go to an emergency department the same day if you stop passing urine, your urine turns dark or bloody, you have new confusion or slurred speech, or you have a rash with bruising and bleeding gums. Also seek care that day if the IV site swells, leaks, or turns pale and painful during an infusion, since acyclovir that escapes the vein can damage the surrounding tissue.
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Common questions
What is the maximum dose of acyclovir injection?
Doses are based on body weight, and the usual adult maximum is 10 mg/kg every 8 hours, which is 30 mg/kg per day. That highest level is reserved for herpes simplex encephalitis and for shingles or chickenpox in people who are immunocompromised. Milder indications use 5 mg/kg every 8 hours. If your kidney function is reduced, the same weight-based dose is given less often, from every 12 hours down to a half dose every 24 hours, and if you are obese the dose is calculated from ideal body weight rather than actual weight.
Can I drink alcohol while I am getting acyclovir injections?
There is no direct chemical interaction between alcohol and acyclovir. The practical problem is fluid: acyclovir needs a good flow of urine to avoid crystallizing in the kidneys, and alcohol makes you lose fluid. Alcohol also adds to the nausea, headache, and dizziness that acyclovir can cause on its own. Skip alcohol while you are on IV treatment and drink water regularly instead, then ask your clinician when it is reasonable to resume.
Is acyclovir injection safe during pregnancy or while breastfeeding?
Acyclovir crosses the placenta, and data from published studies and pregnancy registries have not identified a drug-associated risk of major birth defects. Serious herpes infections in pregnancy, including disseminated infection and encephalitis, are treated with IV acyclovir because leaving the infection untreated carries its own risk to you and the pregnancy. Acyclovir passes into breast milk at concentrations higher than in your blood, though the amount an infant takes in is small. Tell your obstetric clinician you are pregnant or nursing so dosing and monitoring can be planned, and do not feed an infant from a breast that has an active herpes lesion.
Do I need to take acyclovir injection with food?
No. Acyclovir injection goes directly into a vein, so food and meal timing do not affect how much medicine reaches your blood. What does matter is fluid intake, because acyclovir is cleared by the kidneys and needs plenty of urine flow to avoid forming crystals in the kidney tubules. Drink water through the day unless your clinician has restricted your fluids for a heart or kidney reason. If nausea is making it hard to eat or drink, tell your team, since IV fluids can be added to the infusion plan.
What are the signs of getting too much acyclovir?
The two organs that show it first are the kidneys and the brain. Kidney signs include passing much less urine, dark urine, swelling in the legs or hands, and a rise in creatinine on blood tests. Brain and nerve signs include confusion, agitation, hallucinations, tremor, slurred speech, unsteady walking, and in severe cases seizures or coma. These are more likely if you are dehydrated, older, or have reduced kidney function, or if a dose was infused too fast. Call Poison Control at 1-800-222-1222 for a suspected overdose and call 911 for a seizure, collapse, or trouble breathing.
Which medicines interact with acyclovir injection?
Probenecid slows how fast your body clears acyclovir and raises its level in your blood. Zidovudine taken with acyclovir has been linked to deep drowsiness and fatigue. The bigger group to watch is anything else that stresses the kidneys, including aminoglycoside antibiotics such as gentamicin, amphotericin B, cyclosporine, tacrolimus, IV contrast dye, and regular use of NSAIDs like ibuprofen or naproxen, because the combination raises the risk of kidney injury. Give your clinician a full list of prescriptions, over the counter medicines, and supplements before your first infusion so doses and monitoring can be adjusted.