Silymarin

Hepatoprotectant, Antioxidant Flavonolignan complex from Silybum marianum (milk thistle)

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 10-20 mg/kg (of silymarin) q8-12h; or 20-50 mg/kg of milk thistle extract (standardized to 70-80% silymarin) q24h q8-12h Duration: Variable; often long-term (weeks to months) depending on condition
Notes: Use a standardized extract. For acute toxicity (e.g., Amanita), higher doses may be used (e.g., 50 mg/kg q8h) and should be initiated early. Siliphos (phytosome) may be dosed at 5-10 mg/kg q24h.
Cat PO 10-20 mg/kg (of silymarin) q8-12h; or 20-50 mg/kg of milk thistle extract q24h q8-12h Duration: Variable; often long-term
Notes: Cats may be sensitive to the taste; use capsules or mix with food. For hepatic lipidosis, use as adjunct to nutritional support.
Horse PO 2-5 g per 500 kg horse (approximately 4-10 mg/kg) q12-24h q12-24h Duration: Variable; often weeks to months
Notes: Use a high-quality extract. May be administered as a paste or powder in feed.
Cattle PO 10-20 mg/kg q24h (limited data) q24h Duration: Variable
Notes: Not commonly used; limited evidence. Use with caution in food animals due to withdrawal times.
Small Ruminants (sheep, goats) PO 10-20 mg/kg q24h (limited data) q24h Duration: Variable
Notes: Limited evidence; use with caution in food animals.
Rabbit PO 10-20 mg/kg q12-24h (limited data) q12-24h Duration: Variable
Notes: May be administered in a small amount of fruit or vegetable baby food.
Birds/Poultry PO 50-100 mg/kg q24h (empirical) q24h Duration: Variable
Notes: For aflatoxicosis, use as supportive therapy. Mix with feed or water.
Exotic (reptiles, small mammals) PO 10-20 mg/kg q12-24h (empirical) q12-24h Duration: Variable
Notes: Adjust based on species and condition. Use liquid formulations for small patients.

Clinical Indications & Species Uses

General Indications
  • Adjunctive therapy for various liver diseases
  • Antioxidant supplementation
  • Prevention of hepatotoxicity from certain drugs or toxins
Dog (Canine)
  • Hepatoprotection in chronic hepatitis
  • Supportive therapy for hepatic cirrhosis
  • Toxin-induced hepatotoxicity (e.g., Amanita mushroom poisoning, acetaminophen overdose)
  • Cholangiohepatitis
  • Hepatic lipidosis (adjunctive)
  • Antioxidant support in liver disease
Cat (Feline)
  • Hepatic lipidosis (adjunctive therapy)
  • Chronic inflammatory liver disease (e.g., cholangitis)
  • Toxin-induced hepatotoxicity
  • Supportive care for liver failure
Horse (Equine)
  • Supportive therapy for hepatic disease (e.g., hyperammonemia, pyrrolizidine alkaloid toxicity)
  • Antioxidant support in performance horses
Cattle (Bovine)
  • Supportive therapy for hepatic disease (e.g., fatty liver syndrome, toxic hepatopathy)
Small Ruminants (Sheep / Goat)
  • Supportive therapy for hepatic disease (e.g., copper toxicity in sheep)
Rabbit & Small Mammals
  • Hepatic disease support (e.g., hepatic coccidiosis, toxic hepatopathy)
Avian & Poultry
  • Hepatic support in cases of fatty liver hemorrhagic syndrome (FLHS) in layers
  • Supportive therapy for aflatoxicosis
Exotic & Other Species
  • Reptiles: supportive care for hepatic lipidosis
  • Small mammals (e.g., guinea pigs, rats): hepatoprotection in toxic insults

Pharmacology & Mechanism of Action

Drug Class: Hepatoprotectant, Antioxidant | Pharmacological Group: Flavonolignan complex from Silybum marianum (milk thistle)

Mechanism of Action: Silymarin is a mixture of flavonolignans (primarily silybin, silydianin, and silychristin) with antioxidant, anti-inflammatory, and antifibrotic properties. Its hepatoprotective effects are mediated through multiple mechanisms: (1) antioxidant activity by scavenging free radicals and increasing intracellular glutathione levels, (2) inhibition of lipid peroxidation in hepatocyte membranes, (3) stimulation of ribosomal RNA polymerase and protein synthesis, promoting hepatocyte regeneration, (4) modulation of inflammatory cytokines (e.g., TNF-α, interleukins), (5) inhibition of hepatic stellate cell activation, reducing fibrosis, and (6) stabilization of mast cells and inhibition of leukotriene synthesis. Silymarin also inhibits the uptake of toxins (e.g., amanitin from death cap mushrooms) into hepatocytes by competing for bile acid transport systems.

Pharmacodynamics: Silymarin exerts dose-dependent hepatoprotective effects. It reduces liver enzyme elevations (ALT, AST) in various models of hepatotoxicity. It improves bile flow (choleretic effect) and has anti-inflammatory effects in the liver. It also exhibits antifibrotic activity by reducing collagen deposition. In addition, silymarin has been shown to have antioxidant effects in other tissues, including kidney and pancreas, and may have anti-cancer properties in vitro. Its bioavailability is low due to poor water solubility and extensive first-pass metabolism, but formulations with phosphatidylcholine (e.g., siliphos) enhance absorption.

⚡ Pharmacokinetics Summary

Absorption: Silymarin is poorly absorbed after oral administration due to low aqueous solubility. Absorption is enhanced when administered with food or when formulated as a phytosome (e.g., silybin-phosphatidylcholine complex). Peak plasma concentrations occur within 1-3 hours in most species.
Distribution: Silymarin distributes widely, with highest concentrations in the liver, followed by kidneys, lungs, and heart. It crosses the blood-brain barrier to a limited extent. It is extensively bound to plasma proteins (primarily albumin) and bile acids.
Metabolism: Silymarin undergoes extensive first-pass metabolism in the liver. It is metabolized by conjugation (glucuronidation and sulfation) and to a lesser extent by oxidation. The major metabolites are conjugates of silybin and other flavonolignans. Enterohepatic recirculation occurs, prolonging its presence in the body.
Excretion: Excretion is primarily via bile into feces. A small fraction is excreted in urine as metabolites. The elimination half-life is short (1-3 hours in most species) for the parent compound, but metabolites may persist longer.
Half-Life: Dog: ~1.5-2 hours; Cat: ~2-3 hours; Horse: ~1-2 hours; Humans: ~2-4 hours (for silybin)
Bioavailability: Low oral bioavailability (<10% for standard silymarin); enhanced with phytosome formulations (up to 3-5 fold increase).
Protein Binding: Approximately 80-90% bound to plasma proteins, mainly albumin.

Available Formulations & Strengths

Oral Capsule 50 mg, 100 mg, 150 mg, 200 mg, 250 mg, 300 mg (silymarin) (PO)
Oral Tablet 50 mg, 100 mg, 150 mg, 200 mg, 250 mg (PO)
Oral Liquid (tincture or suspension) Various concentrations (e.g., 100 mg/mL) (PO)
Powder (bulk) Standardized to 70-80% silymarin (PO)
Phytosome (Siliphos) Capsule Silybin-phosphatidylcholine complex (e.g., 100 mg silybin) (PO)
Injectable (not common; experimental) N/A (IV/IM (not routinely used))

Contraindications & Clinical Warnings

Contraindications:
  • Known hypersensitivity to silymarin or milk thistle
  • Severe hepatic encephalopathy (use with caution; may require dose adjustment)
  • Pregnancy (limited safety data; use only if benefits outweigh risks)
  • Lactation (limited safety data; use with caution)
Warnings & Clinical Precautions:
  • Use with caution in patients with biliary obstruction (may increase bile flow and exacerbate obstruction)
  • May interfere with cytochrome P450 enzymes (CYP3A4, CYP2C9) and P-glycoprotein; monitor for interactions with other drugs
  • In food animals, observe withdrawal times; not approved for use in food animals in many countries
  • Use with caution in animals with a history of gastrointestinal ulcers (may cause mild GI upset)
  • Not a substitute for standard medical therapy for liver disease; use as adjunctive therapy
  • Ensure adequate hydration and nutritional support in patients with hepatic disease

Adverse Effects & Reactions

Common:

  • Mild gastrointestinal upset (diarrhea, vomiting, anorexia)
  • Flatulence

Serious / Severe:

  • Allergic reactions (rare)
  • Hepatotoxicity (paradoxical, at very high doses)
  • Interference with drug metabolism leading to toxicity of co-administered drugs

Rare:

  • Headache (in humans; not reported in animals)
  • Dermatitis
  • Pruritus

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
CYP3A4 substrates (e.g., ketoconazole, itraconazole, cyclosporine, midazolam) Silymarin may inhibit CYP3A4, increasing plasma levels of these drugs; monitor for toxicity. Moderate
CYP2C9 substrates (e.g., warfarin, phenytoin, tolbutamide) Silymarin may inhibit CYP2C9, increasing drug levels and risk of adverse effects. Moderate
P-glycoprotein substrates (e.g., digoxin, ivermectin, doxorubicin) Silymarin may inhibit P-glycoprotein, increasing absorption and reducing efflux of these drugs, potentially increasing toxicity. Moderate
UDP-glucuronosyltransferase (UGT) substrates (e.g., acetaminophen, morphine) Silymarin may inhibit UGT enzymes, potentially increasing levels of these drugs. Mild
Other hepatotoxic drugs (e.g., NSAIDs, azathioprine, methotrexate) Additive hepatoprotective effects may be beneficial, but monitor for potential interactions. Mild

Overdose & Toxicity Management

Signs of Toxicity:

  • Gastrointestinal upset (vomiting, diarrhea)
  • Lethargy
  • In rare cases, hepatotoxicity at extremely high doses

Emergency Treatment Protocol: Treatment is symptomatic and supportive. Induce vomiting if ingestion is recent and the animal is conscious. Administer activated charcoal to reduce absorption. Provide fluid therapy and supportive care. Monitor liver enzymes and clinical status. There is no specific antidote.

Food Animal Withdrawal Times

🥩 Meat: 0 days🥛 Milk: 0 days🥚 Eggs: 0 days

Silymarin is not approved for use in food animals in many countries. However, due to its low toxicity and rapid elimination, a zero-day withdrawal is often cited, but local regulations should be followed. In the US, extra-label use in food animals requires a withdrawal period established by a veterinarian; a conservative withdrawal of 24 hours for meat and 48 hours for milk may be recommended.

Storage, Handling & Regulatory Information

Storage Temperature: Store at room temperature (20-25°C) in a dry place.

Light Sensitivity: Light-sensitive — protect from direct exposure.

Handling & Special Conditions: Protect from light and moisture. Keep container tightly closed. Do not freeze liquid formulations.

Dispensing Status: Over-The-Counter (OTC)

Approval Status: Not FDA-approved for veterinary use; available as a dietary supplement.

Extra-Label (Off-Label) Use: Silymarin is a dietary supplement and is not FDA-approved for veterinary use. In the US, it is regulated as a supplement, not a drug. Extra-label use in food animals is subject to AMDUCA regulations; a veterinarian must establish a withdrawal period. In the EU, it may be considered a veterinary medicinal product if used for therapeutic purposes; check local regulations.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Silymarin is widely used as a hepatoprotectant in veterinary medicine, particularly for chronic liver disease and toxin exposure. It is generally well-tolerated, but its efficacy is debated due to poor bioavailability. Use a standardized extract (70-80% silymarin) or a phytosome formulation for better absorption. It is often used in combination with other antioxidants (e.g., vitamin E, SAMe) and supportive care. For acute toxicities like Amanita mushroom poisoning, silymarin must be initiated early and may be used at higher doses. It is not a substitute for specific therapies (e.g., antidotes, dietary management) but can be a valuable adjunct. Always monitor liver enzymes and clinical response. In food animals, consider withdrawal times and legal restrictions.

References & Literature

  • 📚 Plumb's Veterinary Drug Handbook
  • 📚 Papich Veterinary Pharmacology and Therapeutics
  • 📚 Compendium of Veterinary Products (CVP)
  • 📚 Veterinary Herbal Medicine (Wynn & Fougère)
  • 📚 Clinical Veterinary Toxicology (Plumlee)