Elephant specific information, if available, is in blue.

Chemistry – A piperazine-derivative antihistamine, hydroxyzine HCl occurs as a white, odorless powder. It is very soluble in water and freely soluble in alcohol. Hydroxyzine pamoate occurs as a light yellow, practically odorless powder. It is practically insoluble in water or alcohol.

Storage/Stability/Compatibility – Hydroxyzine oral products should be stored at room temperature in tight, light-resistant containers. Avoid freezing all liquid products.

The HCl injection has been reported to be compatible with the following drugs when mixed in syringes: atropine sulfate, benzquinamide HCl, butorphanol tartrate, chlorpromazine HCl, cimetidine HCl, codeine phosphate, diphenhydramine HCl, doxapram HCl, droperidol, fentanyl citrate, glycopyrrolate, hydromorphone HCl, lidocaine HCl, meperidine HCl, methotrimeprazine, metoclopramide HCl, midazolam HCl, morphine sulfate, oxymorphone HCl, pentazocine lactate, procaine HCl, prochlorperazine edisylate, promazine HCl, promethazine HCl, and scopolamine HBr. Compatibility is dependent upon factors such as pH, concentration, temperature and diluents used. It is suggested to consult specialized references (e.g., Handbook on Injectable Drugs by Trissel; see bibliography) for more specific information.

Pharmacology – Like other H1-receptor antihistamines, hydroxyzine acts by competing with histamine for sites on H1-receptor sites on effector cells. Antihistamines do not block histamine release, but can antagonize its effects. In addition to its antihistaminic effects, hydroxyzine possesses anticholinergic, sedative, tranquilizing, antispasmodic, local anesthetic, mild bronchodilative, and antiemetic activities.

Uses/Indications – Hydroxyzine is used principally for its antihistaminic, antipruritic and sedative/tranquilization qualities, often in atopic patients.

Pharmacokinetics – Hydroxyzine is rapidly and well absorbed after oral administration. Effects generally persist for 6-8 hours in dogs and up to 12 hours in cats. Hydroxyzine is apparently metabolized in liver.

Contraindications/Precautions/Reproductive Safety – Hydroxyzine is contraindicated in patients hypersensitive to it. It should be used with caution in patients with prostatic hypertrophy, bladder neck obstruction, severe cardiac failure, angle-closure glaucoma, or pyeloduodenal obstruction.

At doses substantially greater than used therapeutically, hydroxyzine has been shown to be teratogenic in lab animals. Use during pregnancy (particularly during the first trimester) only when the benefits outweigh the risks. It is unknown if hydroxyzine enters maternal milk.

Adverse Effects/Warnings – The most likely adverse effect associated with hydroxyzine is sedation. In dogs, this is usually mild and transient. Occasionally antihistamines can cause a hyperexcitability reaction. Dogs have reportedly developed fine rapid tremors, whole body tremors and rarely, seizures while taking this drug. Safe dosages have not been established for cats.

Overdosage/Acute Toxicity – There is limited information available. There are no specific antidotes available. Overdoses would be expected to cause increased sedation and perhaps, hypotension. Gut emptying protocols should be considered with large or un­known quantity overdoses. Supportive and symptomatic treatment is recommended if necessary.

Drug Interactions – Additive CNS depression may be seen if combining hydroxyzine with other CNS depressant medications, such as barbiturates, tranquilizers, etc. Additive anticholinergic effects may occur when hydroxyzine is used concomitantly with other anticholinergic agents. Hydroxyzine may inhibit or reverse the vasopressor effects of epinephrine. Use norepinephrine or metaraminol instead.

Laboratory Considerations – False increases have been reported in 17-hydroxycorticosteroid urine values after hydroxyzine use. Because antihistamines can decrease the wheal and flair response to skin allergen testing, antihistamines should be discontinued from 3-7 days (depending on the antihistamine used and the reference) before intradermal skin tests.

Doses –

Horses:

a)   0.5 – 1 mg/kg IM or PO bid (Robinson 1992)

b)   Using the pamoate salt: 0.67 mg/kg PO twice daily (Duran 1992)

Monitoring Parameters – Efficacy and Adverse Effects

Client Information – May cause drowsiness and impede working dogs’ abilities.

Dosage Forms/Preparations/FDA Approval Status/Withholding Times –

Veterinary-Approved Products: None

Human-Approved Products:

Hydroxyzine HCl Oral Tablets 10 mg, 25 mg, 50 mg, 100 mg; Atarax®  (Roerig), Anxanil®  (film coated tabs) (Econo Med), generic; (Rx)

Hydroxyzine HCl Oral Solution 10 mg/5 ml; Atarax®  (Roerig), generic; (Rx)

Hydroxyzine HCl Injection (for IM use only) 25 mg/ml in 2 ml syringes & 1 and 10 ml vials & 50 mg/ml in 2 ml amps, 1 & 2 ml syringes & 1, 2 and 10 ml vials Vistaril®  (Roerig) is the most commonly known trade name, there are several others including generically labeled products available; (Rx)

Hydroxyzine Pamoate Oral Capsules (equivalent to hydroxyzine HCl) 25 mg, 50 mg, 100 mg; Vistaril®  (Pfizer), generic; (Rx)

Hydroxyzine Pamoate Oral Suspension (equivalent to hydroxyzine HCl) 25 mg/5 ml;Vistaril® (Pfizer); (Rx)