Codeine

Codeine is a weak opioid used empirically in veterinary medicine for cough suppression and selected mild-to-moderate pain states. Its activity depends partly on formation of active metabolites, which varies between species. In dogs, low oral availability and uncertain metabolite contribution make clinical analgesia less predictable than its familiar human use might suggest.

Group

Opioid analgesic and antitussive

Mechanism of Action

Codeine and its active metabolites interact with opioid receptors in the central nervous system. Receptor activation reduces neurotransmitter release and transmission through pain pathways, while central suppression of the cough reflex accounts for its antitussive role. Conversion to morphine and other active metabolites differs between species, so the strength and duration of opioid effects depend on both drug exposure and metabolism.

Dose Rates

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Analgesia	0.5–1 mg/kg	PO	Every 4–6 h
Cough suppression	0.1–0.3 mg/kg	PO	Every 4–6 h
Cough suppression	1–2 mg/kg	PO	Every 6–12 h
Cough suppression	0.1–0.3 mg/kg	PO	Every 8–12 h
Analgesia	0.5–2 mg/kg	PO	Every 6–12 h	Titrate to effect; do not give acetaminophen-containing combinations to cats.
Antidiarrheal use	0.25–0.5 mg/kg	PO	Every 6–8 h	Do not suppress diarrhea from toxin ingestion before toxin clearance; avoid repeated use with severe inflammatory bowel disease.
Higher-dose analgesic alternative	Reference only	PO		The older source gives codeine alone 1–4 mg/kg orally every 1–6 hours, an unusually broad schedule. Preserve for specialist review rather than recommending the shortest interval automatically.
Codeine-acetaminophen analgesia	Reference only	PO		A tablet with 60 mg codeine and 300 mg acetaminophen is described at 1–2 mg/kg CODEINE every 6–8 hours. Do not treat it as combined-total dosing; the codeine salt basis and independently safe acetaminophen exposure must both be established. Never give this combination to cats.

Cats

Use / stage	Dose	Route	Schedule	Instructions
Analgesia	0.5 mg/kg	PO	Every 6 h	Titrate to response; avoid acetaminophen-containing products.
Cough suppression	0.1 mg/kg	PO	Every 6 h
Analgesia	0.5–2 mg/kg	PO	Every 6–12 h	Titrate to effect; do not give acetaminophen-containing combinations to cats.
Analgesia	0.5–2 mg/kg	PO	Every 6–8 h

Rabbits

Use / stage	Dose	Route	Schedule	Instructions
Historical combination elixir in water	Reference only	PO		One mL acetaminophen/codeine elixir in 10–20 mL drinking water is described without ingredient strengths or intake; no dose or preparation is inferred.

Indications

Empirical symptomatic suppression of persistent nonproductive cough in selected dogs after assessment of the underlying respiratory disorder.
Adjunctive treatment of selected mild-to-moderate pain in dogs, sometimes in combination analgesic products; clinical efficacy is incompletely established and oral response is unpredictable.

Contraindications

Do not give cats codeine combinations containing acetaminophen.
Avoid unassessed multi-ingredient cough preparations.
MAO-inhibitor treatment or opioid hypersensitivity.
Diarrhea from uneliminated toxicants or repeated treatment of severe inflammatory bowel disease.
Severe respiratory compromise or increased intracranial pressure requires extreme caution.

Interactions

Other sedatives and CNS depressants increase depression.
Anticholinergics increase constipation risk; antidepressants and other sedatives increase CNS/respiratory depression.
Quinidine can inhibit conversion to morphine, potentially changing analgesic response.

Adverse effects

Sedation
Excitement, especially in cats
Constipation
Dependence with prolonged treatment

Toxicity

Support respiration in severe depression; naloxone reverses serious opioid effects.

Monitoring and Administration

Monitor breathing, heart rate and clinical response. Analgesic response varies and tolerance may require reassessment.

Codeine phosphate and sulfate are different salt formulations. Generic listed strengths do not establish their free-base equivalence, so no unverified salt conversion is used.

The cited canine acetaminophen–codeine example favors the lower 30 mg codeine tablet over 60 mg because of sedation, but this is not a general combination dosing rule.

Listed presentations are 15, 30 and 60 mg tablets and 5 mg/mL syrup or 3 mg/mL solution; their exact salt basis and complete ingredients are not specified, so product conversions are not encoded.