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Source overview
Recorded excerpts for one species. Each dose remains a separate source entry; no regimen is selected.
Choose one species to view its source overview. All-species evidence remains available below.
View full clinical evidence and sources ↓Clinical evidence
Pending clinical review (PROPOSED)Rule-based (RULE_BASED)— applies to the facts and dose entries in this species selection; individual source records retain their own status.
Indications3
Hyperthyroidism (medical management, or pre-surgical/pre-radioiodine stabilization)
CatOn-label (ON_LABEL)1 source record · View provenance
- Cat · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Indications table · Not applicable · On-label (ON_LABEL) · Pending clinical review (PROPOSED) · Rule-based
Original excerpt: Hyperthyroidism (medical management, or pre-surgical/pre-radioiodine stabilization) — Authorized (FDA, Felimazole)
- Cat · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Indications table · Not applicable · On-label (ON_LABEL) · Pending clinical review (PROPOSED) · Rule-based
Felanorm (methimazole) is indicated for the treatment of hyperthyroidism in cats.
Species not recordedOn-label (ON_LABEL)United States2 source records · View provenance
- Species not recorded · SPL section 34067-9 (INDICATIONS & USAGE SECTION) · United States · On-label (ON_LABEL) · Pending clinical review (PROPOSED) · Rule-based
- Species not recorded · SPL section 34067-9 (INDICATIONS & USAGE SECTION) · United States · On-label (ON_LABEL) · Pending clinical review (PROPOSED) · Rule-based
Dose regimens5
This calculator performs arithmetic only on the sourced dosing field shown for each result — it does not verify the field's accuracy, does not select which field to use for a patient, and does not replace a veterinarian's order. Not for clinical use. Confirm every result against the primary source before dispensing.
Choose one species above to view its dose calculator.
Adverse effects114
Vomiting, anorexia, lethargy — Most common, first 3 months, ~10%
CatNot applicable (NOT_APPLICABLE)1 source record · View provenance
- Cat · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Side Effects / Adverse Effects table · Not applicable · Not applicable (NOT_APPLICABLE) · Pending clinical review (PROPOSED) · Rule-based
Eosinophilia, leukopenia, thrombocytopenia, lymphocytosis — ~15% in first 8 weeks, usually transient
CatNot applicable (NOT_APPLICABLE)1 source record · View provenance
- Cat · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Side Effects / Adverse Effects table · Not applicable · Not applicable (NOT_APPLICABLE) · Pending clinical review (PROPOSED) · Rule-based
Self-induced facial excoriation, bleeding, hepatopathy, agranulocytosis, positive ANA — Rare (1.5–2.7% each) but serious — require withdrawal
CatNot applicable (NOT_APPLICABLE)1 source record · View provenance
- Cat · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Side Effects / Adverse Effects table · Not applicable · Not applicable (NOT_APPLICABLE) · Pending clinical review (PROPOSED) · Rule-based
Acquired myasthenia gravis — Rare
CatNot applicable (NOT_APPLICABLE)1 source record · View provenance
- Cat · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Side Effects / Adverse Effects table · Not applicable · Not applicable (NOT_APPLICABLE) · Pending clinical review (PROPOSED) · Rule-based
Unmasking of underlying renal disease — Documented — reducing T4 lowers GFR
CatNot applicable (NOT_APPLICABLE)1 source record · View provenance
- Cat · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Side Effects / Adverse Effects table · Not applicable · Not applicable (NOT_APPLICABLE) · Pending clinical review (PROPOSED) · Rule-based
These are counts of spontaneously reported events, not proven adverse effects, causal relationships, or incidence rates.
Bars compare report counts within each species; they do not show incidence.
More reportsFewer reportsDog
- Anxiety1 reports
- Emesis1 reports
- Itching1 reports
- Lack of efficacy - NOS1 reports
- Urine leakage1 reports
- Vomiting1 reports
- Whining1 reports
Cat
- Vomiting203 reports
- Weight loss144 reports
- Anorexia114 reports
- Lack of efficacy - NOS108 reports
- Lethargy (see also Central nervous system depression in Neurological)105 reports
- Not eating101 reports
- Death by euthanasia89 reports
- Lethargy (see also Central nervous system depression in 'Neurological')86 reports
- Diarrhoea78 reports
- Decreased appetite72 reports
- Ataxia71 reports
- Death68 reports
Monitoring1
Baseline + q2–3 weeks for first 3 months: CBC/platelets, serum T4, renal panel, liver enzymes, ANA if indicated; then T4 every 3–6 months once stable
Species not recordedNot applicable (NOT_APPLICABLE)1 source record · View provenance
- Species not recorded · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Monitoring Parameters · Not applicable · Not applicable (NOT_APPLICABLE) · Pending clinical review (PROPOSED) · Rule-based
Contraindications3
Hypersensitivity to methimazole/carbimazole/polyethylene glycol; autoimmune disease, primary liver disease, renal failure, coagulopathy, pregnant/lactating queens (label)
Species not recordedNot applicable (NOT_APPLICABLE)1 source record · View provenance
- Species not recorded · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Contraindications & Warnings table · Not applicable · Not applicable (NOT_APPLICABLE) · Pending clinical review (PROPOSED) · Rule-based
Do not use in cats with hypersensitivity to methimazole or carbimazole. Do not use in cats with primary liver disease or renal failure. Do not use in cats with autoimmune disease. See ADVERSE REACTIONS. Do not use in cats with hematological disorders (such as anemia, neutropenia, lymphopenia, or thrombocytopenia) or coagulopathies. See ADVERSE REACTIONS. Do not use in pregnant or lactating queens. Laboratory studies in rats and mice have shown evidence of teratogenic and embryotoxic effects of methimazole.
Species not recordedOn-label (ON_LABEL)United States1 source record · View provenance
- Species not recorded · SPL section 34070-3 (CONTRAINDICATIONS SECTION) · United States · On-label (ON_LABEL) · Pending clinical review (PROPOSED) · Rule-based
Do not use in cats with hypersensitivity to methimazole or carbimazole. Do not use in cats with primary liver disease or renal failure. ADVERSE REACTIONS Do not use in cats with autoimmune disease. See. ADVERSE REACTIONS Do not use in cats with hematological disorders (such as anemia, neutropenia, lymphopenia, or thrombocytopenia) or coagulopathies. See. Do not use in pregnant or lactating queens. Laboratory studies in rats and mice have shown evidence of teratogenic and embryotoxic effects of methimazole.
Species not recordedOn-label (ON_LABEL)United States1 source record · View provenance
- Species not recorded · SPL section 34070-3 (CONTRAINDICATIONS SECTION) · United States · On-label (ON_LABEL) · Pending clinical review (PROPOSED) · Rule-based
Precautions9
History of hematologic abnormalities
Species not recordedNot applicable (NOT_APPLICABLE)1 source record · View provenance
- Species not recorded · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Contraindications & Warnings table · Not applicable · Not applicable (NOT_APPLICABLE) · Pending clinical review (PROPOSED) · Rule-based
Human handlers, especially pregnant women, should wear gloves when handling tablets, cat litter, or bodily fluids of treated cats — significant human absorption risk, particularly with transdermal formulation.
Species not recordedNot applicable (NOT_APPLICABLE)1 source record · View provenance
- Species not recorded · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Clinical pearl · Not applicable · Not applicable (NOT_APPLICABLE) · Pending clinical review (PROPOSED) · Rule-based
Twice-daily dosing is more effective than once daily — don't consolidate to simplify owner compliance without a good reason.
Species not recordedNot applicable (NOT_APPLICABLE)1 source record · View provenance
- Species not recorded · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Clinical pearl · Not applicable · Not applicable (NOT_APPLICABLE) · Pending clinical review (PROPOSED) · Rule-based
This drug controls but does not cure hyperthyroidism — set owner expectations for lifelong therapy or definitive treatment (surgery/radioiodine).
Species not recordedNot applicable (NOT_APPLICABLE)1 source record · View provenance
- Species not recorded · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Clinical pearl · Not applicable · Not applicable (NOT_APPLICABLE) · Pending clinical review (PROPOSED) · Rule-based
Methimazole has anti-vitamin K activity and may induce bleeding diathesis without evidence of thrombocytopenia. See ADVERSE REACTIONS. Keep Felanorm in a secure location out of reach of dogs, cats, and other animals to prevent accidental ingestion or overdose.
Species not recordedOn-label (ON_LABEL)United States1 source record · View provenance
- Species not recorded · SPL section 34071-1 (WARNINGS SECTION) · United States · On-label (ON_LABEL) · Pending clinical review (PROPOSED) · Rule-based
Interactions3
Benzimidazole antiparasitics — ↓ hepatic clearance — ↑ blood levels
Species not recordedNot applicable (NOT_APPLICABLE)1 source record · View provenance
- Species not recorded · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Drug Interactions table · Not applicable · Not applicable (NOT_APPLICABLE) · Pending clinical review (PROPOSED) · Rule-based
Digoxin, theophylline, beta-blockers — Dose reduction of these drugs may be needed once cat becomes euthyroid
Species not recordedNot applicable (NOT_APPLICABLE)1 source record · View provenance
- Species not recorded · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Drug Interactions table · Not applicable · Not applicable (NOT_APPLICABLE) · Pending clinical review (PROPOSED) · Rule-based
Warfarin — Anticoagulant effect potentiated — increased monitoring needed
Species not recordedNot applicable (NOT_APPLICABLE)1 source record · View provenance
- Species not recorded · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Drug Interactions table · Not applicable · Not applicable (NOT_APPLICABLE) · Pending clinical review (PROPOSED) · Rule-based
Pharmacokinetics6
Bioavailability — 45–98% (highly variable, PO); ~93% (Papich)
Species not recordedNot applicable (NOT_APPLICABLE)1 source record · View provenance
- Species not recorded · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Pharmacokinetics table · Not applicable · Not applicable (NOT_APPLICABLE) · Pending clinical review (PROPOSED) · Rule-based
Half-life — 2.3–10.2h (cat, variable by source)
Other speciesNot applicable (NOT_APPLICABLE)1 source record · View provenance
- Other species · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Pharmacokinetics table · Not applicable · Not applicable (NOT_APPLICABLE) · Pending clinical review (PROPOSED) · Rule-based
Metabolism — Concentrates in thyroid tissue
Other speciesNot applicable (NOT_APPLICABLE)1 source record · View provenance
- Other species · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Pharmacokinetics table · Not applicable · Not applicable (NOT_APPLICABLE) · Pending clinical review (PROPOSED) · Rule-based
Elimination — Not detailed
Species not recordedNot applicable (NOT_APPLICABLE)1 source record · View provenance
- Species not recorded · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Pharmacokinetics table · Not applicable · Not applicable (NOT_APPLICABLE) · Pending clinical review (PROPOSED) · Rule-based
Methimazole is an antithyroid drug that acts by blocking the biosynthesis of thyroid hormone in vivo. The primary action is to inhibit binding of iodide to the enzyme thyroid peroxidase, thereby preventing the catalyzed iodination of thyroglobulin and T3 and T4 synthesis. Methimazole is well absorbed following oral administration. Maximum plasma concentrations are achieved within 1-1 1/2 hours after dosing and methimazole is rapidly eliminated from the blood (T1/2 is approximately 3 hours). Administration of methimazole in a fasted state enhances absorption.
Species not recordedOn-label (ON_LABEL)United States2 source records · View provenance
- Species not recorded · SPL section 34090-1 (CLINICAL PHARMACOLOGY SECTION) · United States · On-label (ON_LABEL) · Pending clinical review (PROPOSED) · Rule-based
- Species not recorded · SPL section 34090-1 (CLINICAL PHARMACOLOGY SECTION) · United States · On-label (ON_LABEL) · Pending clinical review (PROPOSED) · Rule-based
Pregnancy / lactation considerations1
Contraindicated — crosses placenta, can cause fetal hypothyroidism; teratogenic/embryotoxic in rodent studies. Human FDA category D. Concentrates in milk above plasma levels — place kittens on milk replacer if queen is treated.
Species not recordedNot applicable (NOT_APPLICABLE)1 source record · View provenance
- Species not recorded · Plumb's Veterinary Drug Handbook, 7th ed., p. 800–802; Saunders Handbook of Veterinary Drugs (Papich), 4th ed., p. 506–508 — Pregnancy & Lactation · Not applicable · Not applicable (NOT_APPLICABLE) · Pending clinical review (PROPOSED) · Rule-based
Literature references19
References are linked to this drug. Species applicability is not recorded here, so the species filter does not narrow this list. Imported dates do not retain their original precision; confirm dates in the original citation.
- A predictive algorithm for the optimal daily dosage of thiamazole to control cats with hyperthyroidism.
Study summary not available in the imported record. Open the original citation to review the study.
- Adverse side effects, including agranulocytosis and anemia, from methimazole treatment of a hyperthyroid cat.
Study summary not available in the imported record. Open the original citation to review the study.
- Hyperthyroid cats that develop azotemia following successful radioiodine treatment have shorter survival times compared to cats that remain nonazotemic.
Study summary not available in the imported record. Open the original citation to review the study.
- Fifteen Years of Iodine Prophylaxis in Italy: Results of a Nationwide Surveillance (Period 2015-2019).
Study summary not available in the imported record. Open the original citation to review the study.
- Ex-vivo study of the percutaneous absorption of a tramadol formulation through feline inner pinna skin.
Study summary not available in the imported record. Open the original citation to review the study.
Sources: DailyMed, U.S. National Library of Medicine; PubMed, National Library of Medicine; openFDA, U.S. Food and Drug Administration; Plumb DC. Plumb's Veterinary Drug Handbook. 7th ed. PharmaVet Inc; 2011.
Owner information handout
Plain-language medication information to review with the owner. Open the handout to print it or download a PDF.
Available products
Products listed in the Health Canada Drug Product Database for this active ingredient. A listing does not establish that a product is authorized for the clinical uses described on this page.