HYDROXYZINE HCL, HYDROXYZINE PAMOATE

Chemistry - A piperazine-derivative antihistamine, hydroxyzine HCl occurs as a white, odorlesspowder. It is very soluble in water and freely soluble in alcohol. Hydroxyzine pamoate occurs as alight 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 insyringes: 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 diluentsused. It is suggested to consult specialized references (e.g., Handbook on Injectable Drugs by
Trissel; see bibliography) for more specific information.

Pharmacology - HYDROXYZINE HCL, HYDROXYZINE PAMOATE

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 HCL, HYDROXYZINE PAMOATE

Hydroxyzine is rapidly and well absorbed after oral administration. Effectsgenerally persist for 6-8 hours in dogs and up to 12 hours in cats. Hydroxyzine is apparentlymetabolized 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 thebenefits 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 andrarely, 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 anticholinergiceffects may occur when hydroxyzine is used concomitantly with other anticholinergic agents.
Hydroxyzine may inhibit or reverse the vasopressor effects of epinephrine. Use norepinephrine ormetaraminol instead.
Laboratory Considerations - False increases have been reported in 17-hydroxycorticosteroidurine 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 (dependingon the antihistamine used and the reference) before intradermal skin tests.