Gamithromycin for Cow: Uses, Dosing & Side Effects

This information is for educational purposes only. Never give your pet any medication without your veterinarian's guidance. Dosing, frequency, and safety depend on your pet's specific health profile.

Gamithromycin is a prescription macrolide antibiotic used in cattle. In the United States, the best-known brand is Zactran, a 150 mg/mL injectable solution. It is labeled for beef cattle and non-lactating dairy cattle and is given as a single subcutaneous injection in the neck under your vet's direction.

This drug is designed to concentrate well in lung tissue, which is why it is commonly used when your vet is concerned about bovine respiratory disease (BRD). BRD is a common and costly respiratory illness in calves and feedlot cattle, especially after stressors like weaning, transport, commingling, weather shifts, or recent viral disease.

Gamithromycin is not a general-use antibiotic for every cough or fever. It is a targeted medication with label restrictions, slaughter withdrawal rules, and food-animal safety considerations. Your vet will decide whether it fits the situation, whether testing is needed, and whether another antibiotic or supportive care plan makes more sense.

Gamithromycin is labeled for the treatment of bovine respiratory disease associated with key bacterial pathogens, including Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, and Mycoplasma bovis. In real-world herd medicine, your vet may consider it when cattle have fever, depression, reduced appetite, nasal discharge, increased breathing effort, or other signs that fit BRD.

It is also labeled for the control of respiratory disease in beef and non-lactating dairy cattle at high risk of developing BRD. That means your vet may use it in selected groups shortly after arrival or another high-stress event when the herd's risk profile is high. This is a herd-health decision, not a one-size-fits-all step.

Because BRD can involve viruses, bacteria, stress, and management factors all at once, medication is only part of the plan. Your vet may also recommend temperature monitoring, isolation of sick animals, anti-inflammatory medication, fluids, ventilation changes, vaccination review, and follow-up checks to see whether the group is responding as expected.

For labeled cattle use in the U.S., gamithromycin is given one time at 6 mg/kg body weight, which equals 2 mL per 110 lb of body weight. It is administered under the skin in the neck. If the total volume is more than 10 mL, the dose should be split so that no more than 10 mL is given at one injection site.

Because this is a food-animal medication, accurate weight matters. Underdosing can reduce effectiveness and may contribute to antimicrobial resistance concerns. Overdosing can increase the risk of tissue reactions and residue problems. Your vet may recommend a scale weight, weight tape, or a careful estimate before treatment.

This product has important label restrictions. It is for beef cattle and non-lactating dairy cattle only. It should not be used in female dairy cattle 20 months of age or older, and it should not be used in calves to be processed for veal. The labeled meat withdrawal time is 35 days. Your vet should guide any treatment decision, recordkeeping, and residue-avoidance plan.

The most commonly reported side effects in cattle are brief discomfort during injection and mild to moderate swelling at the injection site. The label also notes that subcutaneous injection can cause a transient local tissue reaction, which may matter at slaughter because it can lead to trim loss of edible tissue.

Call your vet promptly if a treated animal seems to worsen instead of improve, develops marked swelling, has trouble breathing, becomes severely depressed, stops eating, or shows signs that do not fit a routine injection-site reaction. BRD can progress quickly, and a poor response may mean the diagnosis needs to be revisited.

Gamithromycin is also contraindicated in animals previously found to be hypersensitive to the drug. If your herd has a history of reaction to this medication or another macrolide antibiotic, tell your vet before treatment so they can weigh safer options.

Specific cattle interaction data for gamithromycin are limited on the product label, so the safest approach is to have your vet review every medication, vaccine, feed additive, and supplement the animal or group has recently received. That includes anti-inflammatories, other antibiotics, dewormers, and any products used at processing.

Because gamithromycin is a macrolide antibiotic, your vet will usually be thoughtful about how it fits into a broader antimicrobial plan. Combining or closely sequencing antibiotics without a clear reason can make treatment response harder to interpret and may complicate stewardship decisions.

In practice, the biggest safety issues are often not classic drug-drug interactions but treatment overlap, timing, residue compliance, and choosing the right cattle for the drug. Your vet can help decide whether gamithromycin should be used alone, paired with supportive care, or avoided in favor of another option based on age, production class, pregnancy status, and intended market use.