Veterinary Drug Handbook (VDH) is the reference veterinarians turn to when they want an independent source of information on the drugs that are used in veterinary medicine today.

DIMENHYDRINATE

Chemistry - An ethanolamine derivative antihistamine, dimenhydrinate contains approximately54% diphenhydramine and 46% 8-chlorotheophylline. It occurs as an odorless, bitter and numbing-tasting, white crystalline powder with a melting range of 102°-107°C. Dimenhydrinate is slightly soluble in water and is freely soluble in propylene glycol or alcohol. The pH of the commercially available injection ranges from 6.4 to 7.2.

Storage, Stability, Compatibility

Dimenhydrinate products should be stored at room temperature; avoid freezing the oral solution and injectable products. The oral solution should be stored in tight containers and tablets stored in well-closed containers.
Dimenhydrinate injection is reportedly compatible with all commonly used intravenous replenishment solutions and the following drugs: amikacin sulfate, atropine sulfate, calcium gluconate, chloramphenicol sodium succinate, corticotropin, ditrizoate meglumine and sodium, diphenhydramine HCl, droperidol, fentanyl citrate, heparin sodium, iothalamate meglumine and sodium, meperidine HCl, methicillin sodium, metoclopramide, morphine sulfate, norepinephrine bitartrate, oxytetracycline HCl, penicillin G potassium, pentazocine lactate, perphenazine, phenobarbital sodium, potassium chloride, scopolamine HBr, vancomycin HCl and vitamin B-complex w/ vitamin
C.The following drugs are either incompatible or compatible only in certain concentrations with dimenhydrinate: aminophylline, ammonium chloride, amobarbital sodium, butorphanol tartrate, glycopyrrolate, hydrocortisone sodium succinate, hydroxyzine, iodipamide meglumine, pentobarbital sodium, prochlorperazine edisylate, promazine HCl, promethazine HCl, tetracycline HCl, and thiopental sodium. Compatibility is dependent upon factors such as pH, concentration, temperature, and diluents used and it is suggested to consult specialized references for more specific information.

Pharmacology - DIMENHYDRINATE

Dimenhydrinate has antihistaminic, antiemetic, anticholinergic, CNS depressantand local anesthetic effects. These principle pharmacologic actions are thought to be a result of onlythe diphenhydramine moiety. Used most commonly for its antiemetic/motion sickness effects, dimenhydrinate's exact mechanism of action for this indication is unknown, but the drug doesinhibit vestibular stimulation. The anticholinergic actions of dimenhydrinate may play a role inblocking acetylcholine stimulation of the vestibular and reticular systems. Tolerance to the CNSdepressant effects can ensue after a few days of therapy and antiemetic effectiveness also maydiminish with prolonged use.
Uses, Indications - In veterinary medicine, dimenhydrinate is used primarily for its antiemetic effects in the prophylactic treatment of motion sickness in dogs and cats.

Pharmacokinetics - DIMENHYDRINATE

The pharmacokinetics of this agent have apparently not been studied in veterinary species. In humans, the drug is well absorbed after oral administration with antiemetic effects occurring within 30 minutes of administration. Antiemetic effects occur almost immediately after IV injection. The duration of effect is usually 3-6 hours.
Diphenhydramine is metabolized in the liver, and the majority of the drug is excreted as metabolites into the urine. The terminal elimination half-life in adult humans ranges from 2.4 - 9.3 hours.
Contraindications/Precautions - Dimenhydrinate is contraindicated in patients who are hypersensitive to it or to 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 aproblem. Additionally, they should be used with caution in patients with hyperthyroidism, seizuredisorders, cardiovascular disease or hypertension. It may mask the symptoms of ototoxicity andshould therefore be used with this knowledge when concomitantly administering with ototoxicdrugs.

Adverse Effects, Warnings

Most common adverse reactions seen are CNS depression (lethargy, somnolence) and anticholinergic effects (dry mouth, urinary retention). GI effects (diarrhea, vomiting, anorexia) are less common, but have been noted.
The sedative effects of antihistamines, may adversely affect the performance of working dogs. Thesedative effects of antihistamines may diminish with time.
Overdosage - Overdosage may cause CNS stimulation (excitement to seizures) or depression(lethargy to coma), anticholinergic effects, respiratory depression and death. Treatment consists ofemptying the gut if the ingestion was oral. Induce emesis if the patient is alert and CNS status isstable. Administration of a saline cathartic and/or activated charcoal may be given after emesis orgastric lavage. Treatment of other symptoms should be performed using symptomatic andsupportive therapies. Phenytoin (IV) is recommended in the treatment of seizures caused by antihistamine overdose in humans; use of barbiturates and diazepam are avoided.

Drug Interactions

Increased sedation can occur if dimenhydrinate (diphenhydramine) is combined with other CNS depressant drugs. Antihistamines may partially counteract the anticoagulation effects of heparin or warfarin. Diphenhydramine may enhance the effects of epinephrine.
Dimenhydrinate may potentiate the anticholinergic effects of other anticholinergic drugs.
Dimenhydrinate has been demonstrated to induce hepatic microsomal enzymes in animals (species not specified); the clinical implications of this effect are unclear.
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.

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