Methocarbomol

Medication Facts

Performance enhancing: conditionally
Total hours to clear: 24
Frequently used: Most Prescribed
ESC ID: METHOC-0560
Trade names: Robaxin
Drug type: Muscle Relaxer
Plasma half-life · hours: 3 hrs
Equine dosing: 11 to 22 mg/kg IV for initial acute muscle relaxation. 22 to 44 mg/kg administered orally every 12 hours for maintenance.
Cmax · peak plasma concentration: 5 g/mL dose = 5 to 10 mg/mL
Tmax · time to peak plasma concentration: 2-3 hrs
ESC concentration guidance: .01 to 1.0 ng/mL oral dose
Primary withdrawal · hours: 12
Washout withdrawal · hours: 48
ESC control type: B1 - Strictly Controlled - Conditionally PE
Medication report required: MED REPORT REQUIRED

Uses & side effects

Muscle relaxer targeting lower back

Drug interactions

Methocarbamol (commonly sold under the brand name Robaxin) is a skeletal muscle relaxant used in horses to treat muscle spasms, tying-up (exertional rhabdomyolysis), and trauma [1]. While it is generally a safe medication with a wide safety margin, it works by depressing the central nervous system (CNS), making sedatives, tranquilizers, and other CNS depressants its primary drug interaction concern.1. Central Nervous System (CNS) DepressantsCombining methocarbamol with other drugs that cause sedation can result in additive effects, leading to extreme lethargy, profound drowsiness, ataxia (uncoordinated movement, stumbling), or weakness. Use extreme caution or adjust dosages if administering alongside:Alpha-2 Agonists (Sedatives): Detomidine (Dormosedan), Xylazine (Rompun), or Romifidine.Tranquilizers: Acepromazine.Opioids: Butorphanol (Torbugesic).Anesthetics: Any systemic anesthetic agents.2. Anticholinesterase Inhibitors (Myasthenia Gravis Medications)Methocarbamol can inhibit the effects of pyridostigmine bromide and neostigmine. While myasthenia gravis is extremely rare in horses, methocarbamol should not be used in animals suffering from this disease, as it directly antagonizes the therapeutic effects of their medicationSkeletal muscle relaxants in horses interact most critically with sedatives, general anesthetics, and calcium channel blockers, primarily risking dangerous levels of central nervous system (CNS) depression, profound weakness, or acute cardiovascular collapse. Muscle relaxants like methocarbamol (Robaxin), guaifenesin (GGE), and dantrolene are commonly used in equine veterinary medicine to manage exertional rhabdomyolysis ("tying up"), treat acute muscle spasms from injury, or assist with anesthetic induction.Because these drugs alter neuromuscular signals or depress brain and spinal cord pathways, combining them with other veterinary treatments requires close tracking. The key drug interactions are broken down below by drug class:1. Central Nervous System (CNS) Depressants & SedativesMedications: Alpha-2 agonists (Xylazine, Detomidine, Romifidine), Acepromazine, and Benzodiazepines (Diazepam, Midazolam).The Interaction: Centrally acting muscle relaxants like methocarbamol and guaifenesin naturally cause mild tranquilization as a side effect. Giving them alongside standard equine sedatives produces a heavy, additive sedative effect.Clinical Impact: High. The horse may experience severe ataxia (unsteadiness), profound muscle weakness, or can become recumbent (unable to stand). This combination compromises the horse's ability to safely maintain its balance.2. General Anesthetics (The "Triple Drip" Effect)Medications: Ketamine, Propofol, and inhalant gases (Isoflurane).The Interaction: Intravenous guaifenesin is routinely combined with ketamine and xylazine (a mixture known as "triple drip") to smoothly induce or maintain short-term anesthesia in horses.Clinical Impact: Intended but narrow safety margin. While this is a common therapeutic technique, precise dosing is vital. If the horse receives excessive guaifenesin, the muscle relaxation overrides the respiratory muscles, resulting in severe respiratory depression, hypoxemia, or an inability to stand during the recovery phase.3. Calcium Channel Blockers (with Dantrolene)Medications: Diltiazem, Amlodipine.The Interaction: Dantrolene is a peripherally acting muscle relaxant that works by directly blocking calcium release inside muscle cells to prevent cramping. Calcium channel blockers similarly restrict calcium movement in cardiac and blood vessel tissues.Clinical Impact: Critical risk / Contraindicated. Combining dantrolene with a calcium channel blocker can cause severe cardiovascular collapse, profound myocardial depression (heart failure), and dangerously high potassium levels in the blood (hyperkalemia).4. Overlapping Muscle Relaxants (Methocarbamol + Guaifenesin)Medications: Concurrent use of oral/IV methocarbamol and IV guaifenesin.The Interaction: Methocarbamol is chemically related to guaifenesin; in fact, guaifenesin is a direct metabolic byproduct of methocarbamol breakdown in the horse's body.Clinical Impact: High regulatory and toxicity risk. Using both simultaneously exponentially increases the risk of overdosage, leading to severe weakness, recumbency, and muscle tremors. For competitive horses, this combination significantly extends drug clearing times, risking positive drug tests under USEF or racing regulations.

Contraindications

Gabapentin, Detomidine (Dormosedan), Xylazine (Rompun), or Romifidine, Acepromozine, Butorphanol (Torbugesic)

Products & routes

Robaxin Injectable (human-labeled product)

Injectable solution · 100 mg methocarbamol/mL (1,000 mg per 10 mL vial)

IV, IM

Product label / source

Veterinary guidance

Dosing and pharmacokinetic data are sourced from publicly available resources, and accuracy is not guaranteed. The dosing samples provided are for illustrative purposes only and should not be used for actual administration; they assume a single medication is delivered at the correct individual dose. Loading doses, or combinations with other substances to increase efficacy or mask concentration levels, are strictly prohibited. Veterinarians must prescribe all medications and determine appropriate withdrawal times based on the horse's health, administration route, drug strength, and timing. The health of the horse must always take priority over performance.

Medication combinations

Combining certain substances or use of can trigger dangerous interactions or contraindications, posing severe health risks or creating unintended performance-enhancing effects. If a medical treatment causes these interactions or alters performance, the horse must be withdrawn from competition until its system clears, or face an adverse drug test.

Government Numbers

Robaxin Injectable: human prescription product; NDA 011790; NDC 0641-6103-01 (10 mL vial). Labeler: Hikma Pharmaceuticals USA Inc. Routes shown are those on the human product label, not a determination of equine administration suitability. These identifiers do not establish equine approval or an equine dose. Reviewed September 18, 2026. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=090a3d3d-92c3-4c30-a648-436461f81702

Sanctioning-body comparison

Plasma only. Compare published competition guidance for this medication. ESC guidance remains in Medication Facts above. A dosing guideline, withdrawal recommendation and laboratory detection threshold are different measures.

Sources reviewed September 23, 2026. “Not verified” means a gap in this comparison, not zero tolerance or permission. Doses describe the cited rules; your veterinarian must determine treatment and withdrawal.

Published guidance by organization
Organization	Published dose & route	Plasma concentration	Withdrawal / timing	Control / penalty class
AQHA Show HorsesShow rules · 2026 handbook Conditions & sources VIO405.13 Reviewed 2026-09-23 2026 AQHA Handbook · VIO400–406 Current AQHA rulebook	5 mg/lb oral or IV per 24 h (5 g/1,000 lb); up to 5 days.	0.5 µg/mL	12 h; no repeat dose within 24 h.	Quantitatively restricted therapeutic.
APHA Paint Show HorsesShow rules · 2026 Rule Book Conditions & sources An unverified entry is not permission to use this medication. Consult the linked current rules. Reviewed 2026-09-23 2026 APHA Rule Book · SC-085, pp. 75–80 Current APHA rulebook	Not verified for this medication.	Not verified for this medication.	Not verified for this medication.	Medication-specific control not yet verified.
ARCI Model RulesRacing model · Rules 15 / UCG 20.0 / CTMS 4.2.1 Conditions & sources ARCI model comparison only; no state-specific rule is substituted. Schedule timing is tied to its stated regimen and is not a clearance guarantee. CTMS 4.2.1; UCG 20.0 PDF p. 38 Reviewed 2026-09-23 ARCI UCG 20.0 · April 2026 ARCI CTMS 4.2.1 · December 2020 ARCI Model Rules 15 · August 2026	15 mg/kg IV once or 5 g orally	1 ng/mL	48 h withdrawal guideline	Drug Class 4 · Penalty Class C. Controlled therapeutic schedule; classification shown below.
USEFNational Therapeutic Substance Provisions · 2026 + July amendment Conditions & sources July 27, 2026 amendment supersedes the booklet’s older 12 h-only advice. GR410.5a; August 10, 2026 supplement Reviewed 2026-09-23 2026 USEF Drugs & Medications Guidelines · August 10 update USEF July 27, 2026 update	5 mg/lb oral or IV per 24 h (5 g/1,000 lb); up to 5 days.	0.5 µg/mL	12 h alone without supplements; 24 h if any other medication or supplement was given within 24 h before competing.	Restricted therapeutic under GR410.5.