Amprolium is a medication used to prevent and treat coccidiosis, a parasitic intestinal disease, in poultry and cattle. It works by mimicking vitamin B1 (thiamine) and starving the parasite of a nutrient it needs to survive. Amprolium is one of the most widely used anticoccidial drugs in livestock, available over the counter as an oral solution or soluble powder that you mix into drinking water.
How Amprolium Works
Coccidia are single-celled parasites that invade the intestinal lining of chickens, turkeys, and calves. They need thiamine (vitamin B1) to fuel their metabolism, just like any living organism. Amprolium has a chemical structure almost identical to thiamine, so the parasite absorbs it readily. But amprolium is missing a key chemical group that thiamine carries, which means once it’s inside the parasite, it can’t actually perform any of thiamine’s metabolic jobs. The parasite essentially fills up on a nutrient lookalike that does nothing, blocking the real thiamine from getting in.
This trick is remarkably selective. The parasite’s thiamine transport system is about 50 times more sensitive to amprolium’s interference than the chicken’s own system. That huge gap is why amprolium can wipe out coccidia at doses that leave the host animal largely unaffected. Beyond starving the parasite of thiamine, amprolium also disrupts the formation of the tough outer wall coccidia use to spread through the environment and inhibits their ability to mature into an infectious form.
What It’s Approved to Treat
In the United States, amprolium is FDA-approved for treating coccidiosis in growing chickens, turkeys, laying hens, and calves. It holds a unique distinction among anticoccidial drugs: it is the only one approved for use in laying hens. This matters because the other major class of coccidiosis drugs, ionophore antibiotics, are toxic to layers and can cause problems ranging from bleached eggshells to reduced egg production and poor hatchability.
Amprolium is also used off-label for other livestock species, including goats and sheep, though those uses don’t carry formal FDA approval.
How Amprolium Compares to Ionophores
The two main categories of coccidiosis drugs are synthetic compounds like amprolium and ionophore antibiotics. Ionophores work by disrupting the balance of ions and water inside the parasite’s cells, which kills them but also puts treated birds at higher risk of heat stress in hot, humid conditions. Amprolium’s thiamine-blocking approach avoids that problem entirely.
Amprolium also has a much wider safety margin. It can be given at five times the recommended dose before toxic effects appear, which is unusually forgiving for a livestock medication. Ionophores, by contrast, have a narrower window between an effective dose and a harmful one, and accidental overdose can be fatal in some species, particularly horses.
Available Forms and How It’s Given
Amprolium comes in two main commercial forms: a 9.6% oral solution and a 20% soluble powder. Both are mixed into drinking water. The liquid solution is sold under brand names like Corid and AmproMed-P, while the powder is typically marketed as Corid 20% Soluble Powder. You’ll find these at farm supply stores without a prescription.
For poultry, the standard treatment protocol calls for mixing amprolium into drinking water at a 0.012% concentration (8 fluid ounces of the 9.6% solution per 50 gallons of water) as soon as coccidiosis is diagnosed, continuing for three to five days. In severe outbreaks, you can double that to 0.024%. After the initial treatment period, you drop to a lower maintenance level of 0.006% for one to two additional weeks.
For calves, there are two protocols. A five-day treatment course uses 10 mg of amprolium per kilogram of body weight daily, delivered either through medicated water or as a direct oral drench. A 21-day prevention protocol uses half that dose, 5 mg per kilogram daily, during periods when calves are likely to be exposed. In both cases, the medicated water should be the animal’s only water source for the full treatment period.
Withdrawal Periods for Meat and Eggs
One of amprolium’s practical advantages is its minimal withdrawal requirements. When chickens are treated at label doses, there is no required withdrawal period for either meat or eggs. You can continue collecting and consuming eggs during treatment. For calves, there is a seven-day withdrawal period: treated animals cannot be slaughtered for food until at least seven days after the last dose.
Risks of Overuse and Thiamine Deficiency
Because amprolium works by blocking thiamine, excessive doses or prolonged treatment can deplete thiamine in the treated animal itself, not just the parasite. At recommended doses and durations, the drug’s selectivity keeps this from being a problem. But high doses or extended courses can tip the balance.
In cattle, sheep, and goats, amprolium-induced thiamine deficiency can cause a serious brain condition called polioencephalomalacia, which involves swelling and death of brain tissue. Affected animals may show blindness, incoordination, head pressing, and seizures. In dogs and cats, severe thiamine depletion causes a condition known as Chastek’s paralysis.
Animal studies have documented what happens when thiamine depletion becomes significant. Mice given amprolium at high doses showed a nearly 50% drop in locomotion and exploratory activity, along with reduced motor coordination. Their brain cells showed about a 10% decrease in viability in the cerebral cortex, and their livers developed fatty changes visible on examination. These animals also lost weight and ate less. The takeaway is straightforward: amprolium is safe within its labeled dose range and treatment duration, but pushing beyond those boundaries creates real harm through the same thiamine-blocking mechanism that makes it effective against coccidia.
Storage and Shelf Life
Unopened amprolium products have a shelf life of about two years when stored below 86°F (30°C). Once you open the container, you should use it within four months. The mixed, diluted solution in your waterer is only stable for 24 hours, so you need to prepare fresh medicated water daily. Store the product in its original container, away from direct sunlight and heat.