Corid is an over-the-counter veterinary medication used to treat and prevent coccidiosis in calves. Its active ingredient is amprolium (9.6% in the liquid form, 20% in the soluble powder), and it targets two specific species of coccidia parasites: Eimeria bovis and E. zurnii. First approved by the FDA in 1962, Corid remains one of the most widely used tools for managing this common and potentially deadly intestinal disease in young cattle.
What Coccidiosis Does to Calves
Coccidiosis is a parasitic infection of the intestinal lining. Calves pick up coccidia oocysts (essentially parasite eggs) from contaminated water, feed, or soil, and the parasites multiply inside the intestinal cells. The result is damage to the gut wall that causes watery or bloody diarrhea, dehydration, weight loss, and in severe cases, death. Calves between three weeks and six months old are most vulnerable, especially during stressful events like weaning, transport, or overcrowding.
The disease spreads quickly in group housing or on pastures with high stocking density. By the time you see bloody stool in one calf, others in the group have likely already been exposed, which is why prevention matters as much as treatment.
How Amprolium Works
Amprolium mimics thiamine (vitamin B1) closely enough to fool the parasite, but it’s structurally different in one critical way: it lacks a chemical group that thiamine needs to function inside cells. When coccidia absorb amprolium instead of real thiamine, they can’t carry out the energy-producing processes they need to grow and reproduce. The parasites’ thiamine transport system is roughly 50 times more sensitive to this interference than the host animal’s, which is why amprolium can starve the parasite without harming the calf at recommended doses.
Beyond blocking nutrient uptake, amprolium also disrupts the parasite’s ability to form normal oocyst walls and complete its reproductive cycle. This means fewer viable oocysts shed into the environment, which helps reduce reinfection pressure across the herd.
Treatment vs. Prevention Protocols
Corid is used in two distinct ways depending on the situation: a higher-dose short course for active infections, and a lower-dose extended course for prevention during high-risk periods.
5-Day Treatment
For calves already showing signs of coccidiosis, the target dose is 10 mg of amprolium per kilogram of body weight daily for five consecutive days. Using the 9.6% liquid solution, you add 16 fluid ounces to every 100 gallons of drinking water. This medicated water must be the only water source for the full five days. Alternatively, you can drench individual calves directly: mix 3 fluid ounces of the 9.6% solution into 1 pint of water, then give 1 fluid ounce of that mixture per 100 pounds of body weight daily.
For the 20% soluble powder, the equivalent is 4 ounces of powder per 50 gallons of drinking water, or 3 ounces dissolved in 1 quart of water for drenching at the same 1 fluid ounce per 100 pounds rate.
21-Day Prevention
When coccidiosis exposure is expected or historically common on your operation, the prevention dose is 5 mg per kilogram of body weight daily for 21 days. With the 9.6% liquid, that’s 8 fluid ounces per 100 gallons of drinking water. As a drench, mix 1.5 fluid ounces into 1 pint of water and give 1 fluid ounce per 100 pounds of body weight. The 20% powder equivalent is 4 ounces per 100 gallons of water, or 1.5 ounces per quart for drenching.
The prevention protocol is commonly started before anticipated stress events, like moving calves to a new pasture or co-mingling groups, when parasite loads in the environment tend to spike.
Available Formulations
Corid comes in two formulations. The 9.6% oral solution is a liquid concentrate, convenient for mixing into water tanks or for precision drenching. The 20% soluble powder dissolves in water and works well with automatic water proportioners. Both deliver the same active ingredient at the same target doses, so the choice comes down to your setup and preference. The powder formulation includes instructions for stock solutions compatible with proportioners that meter 1 fluid ounce per gallon of drinking water.
Risks of Overuse or Overdose
Because amprolium works by blocking thiamine, the main risk of excessive dosing is thiamine deficiency in the animal itself. At recommended levels this isn’t a concern, since the drug is far more selective for the parasite. But at dramatically elevated doses, the same mechanism can affect the calf’s brain.
In a study on buffalo calves, researchers induced a neurological condition called polioencephalomalacia by drenching amprolium at 300 mg per kilogram daily, roughly 30 times the standard treatment dose, for four to six weeks. The calves developed measurable drops in a thiamine-dependent enzyme and rising levels of lactate and pyruvate in their blood and spinal fluid, both markers of impaired energy metabolism in the brain. The takeaway is straightforward: follow the labeled dose and duration. At normal levels, Corid has a wide safety margin, but prolonged or extreme overdosing can cause serious neurological damage.
Regulatory and Practical Details
Corid is classified as an over-the-counter product, so it does not require a prescription or a Veterinary Feed Directive. You can purchase it at most farm supply retailers. It is labeled specifically for beef and dairy calves. While many livestock owners use amprolium off-label in goats, sheep, and poultry, the Corid product label is approved only for cattle.
One practical consideration: because the medicated water must be the sole water source during treatment, you need to ensure calves can’t access any other water. In pasture situations, this sometimes means temporary confinement or using troughs that can be monitored. During hot weather, consumption rates rise, so some producers switch to the drench method for more precise dosing when temperatures make water intake unpredictable.