Chemistry - An ethanolamine-derivative antihistamine, doxylamine succinate occurs as a white to creamy-white powder with a characteristic odor. It has a melting range of 103-108°C and pKa values of 5.8 and 9.3. Doxylamine succinate has solubilities of 0.5 g/ml in alcohol and 1 g/ml in water. The commercially available injection has an approximate pH of 4.8 - 5.2.
Storage/Stability/Compatibility - Tablets should be stored in well-closed, light-resistant packaging at room temperature. No information on the storage, stability, or compatibility was found regarding the injectable product.
Pharmacology - Like other antihistamines, doxylamine competitively inhibits histamine at H1 receptors. It also has substantial sedative and anticholinergic effects.
Uses/Indications - This drug is recommended (by the manufacturer) “for use in conditions in which antihistaminic therapy may be expected to alleviate some signs of disease in dogs, cats, and horses.”
Pharmacokinetics - Pharmacokinetic parameters are apparently unavailable for domestic animals. Doxylamine has a serum half-life of approximately 10 hours in human adults.
Contraindications/Precautions - The manufacturer recommends that the injectable product should not be administered by the IV route in dogs or cats, and that IM and SQ injection sites be divided. Inject slowly IV in horses. Do not use in horses intended for food purposes.
Doxylamine is also contraindicated in patients who are hypersensitive to it or other antihistamines in its class. Because of their anticholinergic activity, antihistamines should be used with caution in patients with angle closure glaucoma, prostatic hypertrophy, pyloroduodenal or bladder neck obstruction, and COPD if mucosal secretions are a problem. Additionally, they should be used with caution in patients with hyperthyroidism, cardiovascular disease or hypertension.
Adverse Effects/Warnings - The manufacturer lists CNS depression, incoordination and GI disturbances as adverse effects at therapeutic dosages.
Overdosage - The manufacturer includes CNS stimulation (excitement, seizures) and ataxia as symptoms associated with overdosage. Treatment should be supportive, as outlined in the previous antihistamine monographs.
Drug Interactions - Potential drug interactions for doxylamine include, increased sedation if doxylamine is combined with other CNS depressant drugs.
Antihistamines may partially counteract the anticoagulation effects of heparin or warfarin.
Doxylamine may enhance the effects of epinephrine.
Laboratory Interactions - Antihistamines can decrease the wheal and flare response to antigen skin testing. In humans, it is suggested that antihistamines be discontinued at least 4 days before testing.
a) 0.55 mg/kg IV(slowly), IM or SQ q8-12h prn. For maintenance therapy: 2.2 - 4.4 mg/kg/day PO divided into 3 or 4 daily doses. (Package Insert; A-H® Injection - Coopers Animal Health)
b) 0.55 mg/kg q8-12h IM or SQ (Schultz 1986)
Monitoring Parameters -
1) Efficacy/Adverse effects
Dosage Forms/Preparations/FDA Approval Status -
Doxylamine Succinate for Injection 11.36 mg/ml; 250 ml vials; A-H® Injection (Schering); (Rx) Approved for use in dogs, cats, and horses (not intended for food purposes).
Doxylamine Succinate Tablets 25 mg, 100 mg; bottles of 50; A-H® Tablets (Schering); (Rx) Approved for use in dogs, cats, and horses (not intended for food purposes).
Human-Approved Products: None
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