The standard dose of dexamethasone for a horse is 0.04 to 0.05 mg/kg given once daily. For a typical 1,100-pound (500 kg) horse, that works out to roughly 20 mg per day. The exact amount depends on the route of administration, whether the horse has eaten recently, and what condition you’re treating.
Standard Dosing by Route
Dexamethasone can be given intravenously (IV), intramuscularly (IM), or orally. The baseline dose stays similar across routes, but oral dosing requires some adjustment because the drug isn’t fully absorbed through the gut.
- IV or IM: 0.04 mg/kg, once every 24 hours. For a 500 kg horse, that’s about 20 mg.
- Oral: 0.05 mg/kg, once every 24 hours. The slightly higher oral dose compensates for lower absorption.
Most injectable dexamethasone sodium phosphate comes in vials labeled 4 mg/mL, but each milliliter actually delivers the equivalent of 3 mg of active dexamethasone. That distinction matters when you’re calculating volumes. For a 500 kg horse getting 20 mg IV, you’d draw up roughly 6 to 7 mL from a standard 4 mg/mL vial.
Why Feeding Status Matters for Oral Doses
Oral dexamethasone absorption varies significantly depending on whether the horse has eaten. Research from the American Journal of Veterinary Research found that bioavailability of oral dexamethasone powder was 66% in fasted horses but dropped to just 28% in horses that had been fed. Oral dexamethasone solution showed a similar pattern: 42% bioavailability on an empty stomach versus 31% after eating.
This means a fed horse may absorb less than half the drug compared to a fasted horse given the same dose. If you’re giving dex orally, administering it before the morning meal improves how much actually reaches the bloodstream. Powdered dexamethasone given to an unfed horse produced the highest peak blood levels of any oral formulation tested.
Higher Doses for Airway Obstruction
Horses with recurrent airway obstruction (commonly called heaves) sometimes need higher doses than the standard range. Studies on horses with active airway obstruction used 0.1 mg/kg IV, which is roughly double the typical anti-inflammatory dose, and saw meaningful improvements in lung function within four hours.
Oral dosing for heaves ran even higher. Researchers found that 0.164 mg/kg given orally to fasted horses produced a peak effect at 24 hours. When the same dose was given to fed horses, the effect was roughly equivalent to giving half that amount (0.082 mg/kg) to fasted horses. This again underscores how much feeding blunts oral absorption. For a 500 kg horse with heaves, these protocols translate to 50 mg IV or up to 82 mg orally, well above the standard maintenance dose and only appropriate under veterinary guidance for confirmed respiratory disease.
For long-term asthma management, the Merck Veterinary Manual lists 0.04 mg/kg IV or IM and 0.05 mg/kg orally as the ongoing daily doses, with the expectation that environmental management (reducing dust, improving ventilation) will also be addressed.
Laminitis and Other Risks
The biggest concern with dexamethasone in horses is laminitis. Corticosteroids raise blood sugar and insulin levels, and in susceptible horses, that hormonal shift can trigger inflammation in the hoof laminae. Higher doses increase systemic exposure and raise the risk of these metabolic side effects.
Horses with Pituitary Pars Intermedia Dysfunction (PPID, often called Cushing’s disease) are at particular risk. These horses already have disrupted hormone regulation, and adding a potent corticosteroid on top of that can push them toward a laminitic episode. Even a single dose of dexamethasone used for diagnostic testing in PPID horses has been associated with laminitis.
Dexamethasone also suppresses cortisol production. After a single dose given either orally or IV, cortisol levels dropped below baseline for 48 to 72 hours in fasted horses and 24 to 48 hours in fed horses. Repeated or prolonged use without a tapering schedule can leave the adrenal glands sluggish, so abrupt withdrawal after extended treatment is not recommended.
Competition Withdrawal Times
Dexamethasone is a controlled substance under competition rules. The FEI lists a detection time of 48 hours (two days) after a single 10 mg IV dose. However, detection time and withdrawal time are not the same thing. The detection time tells you how long the lab can find the drug; the withdrawal time adds a safety margin on top of that to account for individual variation in how horses metabolize drugs.
Your veterinarian will set the actual withdrawal time, and it will be longer than 48 hours. USEF rules follow a similar framework but may differ in specifics. If you’re competing, confirm the current withdrawal guidelines with your governing body before administering dex, because a positive test result carries penalties regardless of whether the drug was given for legitimate medical reasons.
Quick Reference by Horse Weight
These figures use the standard 0.05 mg/kg oral dose and 0.04 mg/kg injectable dose for general anti-inflammatory use:
- 400 kg (880 lb) horse: 16 mg IV/IM or 20 mg oral
- 500 kg (1,100 lb) horse: 20 mg IV/IM or 25 mg oral
- 600 kg (1,320 lb) horse: 24 mg IV/IM or 30 mg oral
Accurate dosing starts with knowing your horse’s actual weight. Guessing can lead to underdosing (ineffective treatment) or overdosing (increased laminitis risk). A livestock scale or a calibrated weight tape gives you a reliable number to calculate from.