Zidovudine (azt) in Cats

Zidovudine, also called AZT, is an antiviral medication your vet may prescribe for some cats with feline immunodeficiency virus (FIV) or feline leukemia virus (FeLV). It is not a cure for either infection. Instead, it is used in selected cases to reduce viral replication and help manage certain clinical problems linked to these retroviruses. In practice, it is most often discussed for cats with FIV-related mouth inflammation such as gingivitis or stomatitis, and for some cats with neurologic signs that may be associated with FIV.

This medication is used off label in cats, which is common in veterinary medicine. That means your vet is using a human-labeled drug in a way supported by veterinary evidence and clinical experience. The decision to use zidovudine depends on the cat’s diagnosis, current symptoms, blood work, and overall goals of care. Some cats may benefit from a trial, while others may be poor candidates because the drug can suppress bone marrow and cause anemia.

For FeLV, the evidence is more limited in naturally infected pet cats than many pet parents expect. Research summarized by Merck notes that zidovudine may reduce viral replication and was most effective when given very early after exposure in experimental settings, but those results have not been consistently replicated in naturally occurring FeLV infection. That is why your vet may frame zidovudine as one option within a broader care plan rather than a stand-alone answer.

The biggest practical point is monitoring. Cats taking zidovudine need baseline and repeat complete blood counts, because anemia and other marrow-related effects are the main safety concern. If your cat is already weak, pale, or has known bone marrow disease, your vet may recommend a different path or a more cautious monitoring schedule.

Zidovudine is a nucleoside reverse transcriptase inhibitor, often shortened to NRTI. It works by interfering with reverse transcriptase, the enzyme retroviruses use to copy their genetic material into DNA inside infected cells. By blocking that step, zidovudine can reduce viral replication. In cats, that matters because both FIV and FeLV are retroviruses.

Merck describes zidovudine as a thymidine analogue that gets incorporated during viral DNA transcription, which stops the replication process. Because the drug acts early in viral replication, it tends to have more effect on actively replicating virus than on cells that are already chronically infected. That helps explain why the medication may improve some clinical signs without eliminating infection.

In real-world feline medicine, the goal is usually symptom control and quality of life, not viral eradication. Cornell notes that zidovudine can help some FIV-positive cats with severe stomatitis or neurologic disease, but it has not been shown to prolong survival in FIV-infected cats. That is an important expectation-setting point for pet parents. A cat may feel better on the medication, yet still need ongoing dental, nutritional, infection-control, and supportive care.

Your vet may also weigh timing and disease stage. Experimental FeLV work suggests earlier use may reduce replication more effectively than delayed treatment, but naturally infected household cats are often diagnosed later in the course of disease. Because of that, the medication is usually one tool among several, not a complete treatment plan by itself.

The most important side effect with zidovudine in cats is bone marrow suppression. In practical terms, that means the drug can lower blood cell production and lead to anemia, and in some cases neutropenia. Merck specifically warns that zidovudine may cause Heinz body anemia in cats, and VCA recommends baseline and periodic complete blood counts for that reason.

Pet parents may first notice vague signs rather than a lab abnormality. Watch for lethargy, weakness, pale gums, poor appetite, or a drop in activity. VCA also lists diarrhea and weakness as possible side effects, and notes that some cats may vomit or act sick if the medication is given on an empty stomach. In older studies of FeLV-infected cats, anorexia, vomiting, and icterus were also reported during treatment.

Some cats tolerate zidovudine reasonably well when carefully selected and monitored. Others do not. Risk tends to be higher in cats that already have kidney disease, liver disease, or bone marrow problems, and in cats taking other medications that can also suppress marrow function. If your cat develops pale gums, marked lethargy, facial swelling, trouble breathing, or any sudden decline, stop the medication only if your vet has instructed you to do so and contact your vet right away. See your vet immediately if your cat seems weak enough to collapse or has trouble breathing.

Because side effects can develop after treatment has already started, follow-up matters as much as the first prescription. A cat may look stable at home while blood counts are drifting downward. That is why your vet may recommend repeat CBC testing within days to weeks after starting therapy, then at intervals based on your cat’s response and other health issues.

Zidovudine dosing in cats should be set by your vet, not estimated at home. Merck’s current dosing table lists 5 to 10 mg/kg by mouth or subcutaneously every 12 hours for cats. VCA notes that the drug comes in capsule, tablet, liquid suspension, and injectable forms, with oral forms used most commonly at home. Because cats are small and doses often need to be precise, compounded liquid formulations are frequently used.

Your vet may choose the lower end of the range when there is concern about side effects, especially in FeLV-positive cats or cats with borderline blood counts. The ABCD FeLV guideline summary available through Merck-linked literature also supports caution with dosing because marrow toxicity is a real concern. Dose changes may be needed if your cat develops anemia, if the hematocrit drops, or if there is kidney or liver disease that could affect drug handling.

Zidovudine may be given with or without food. If your cat vomits after getting it on an empty stomach, your vet may suggest giving future doses with a small meal or treat. If you miss a dose, VCA advises giving it when remembered unless it is close to the next scheduled dose. Do not double up. Consistent timing matters because the medication is usually prescribed every 12 hours.

Do not crush, split, or reformulate the medication unless your vet or pharmacist tells you it is safe for that specific product. Storage instructions vary by form, but VCA advises room-temperature storage for oral products and protection from light and moisture. If your cat resists pilling, ask your vet whether a flavored compounded liquid, capsule, or alternate administration plan would be more realistic for your household.

Zidovudine has several important interaction concerns, mostly because anything else that stresses the bone marrow can raise the risk of anemia or low white blood cells. VCA lists antineoplastic drugs, chloramphenicol, nitrofurantoin, methotrexate, leflunomide, methimazole, and other myelosuppressive medications among the drugs that should be used with caution. Merck also notes that the risk of bone marrow suppression increases with drugs that inhibit glucuronidation or renal excretion.

Other medications may not be absolutely prohibited, but they can change the risk-benefit balance. VCA also advises caution with benzodiazepines, certain antifungals such as fluconazole and amphotericin B, other antivirals such as acyclovir, and steroids including prednisolone and dexamethasone. In some cats, your vet may still choose combinations if the clinical need is strong, but that usually means closer monitoring.

This is one reason a full medication list matters. Tell your vet about prescription drugs, over-the-counter products, supplements, probiotics, herbal products, and any recent injections. Even if something seems unrelated, it may affect liver handling, kidney clearance, appetite, or blood counts. If your cat sees more than one clinic, make sure each team has the same current medication list.

Never start, stop, or add another medication to a cat taking zidovudine without checking with your vet first. That includes leftover antibiotics, pain medications from a prior illness, and human medications kept at home. Cats are sensitive to dosing errors, and the interaction risk with zidovudine is too important to guess.