Diphenhydramine represents an antihistamine that is primarily used to treat allergies, even though it is also very effective as a sleep aid. It is available without a prescription, so visiting the veterinarian is not mandatory prior to purchasing this drug. Diphenhydramine hydrochloride is also available in an injectable form.
Pharmacological Mechanism of Diphenhydramine
As mentioned before, Diphenhydramine is an antihistamine, which means that it blocks the effect of this compound. Histamine acts as a neurotransmitter and takes part in local immune responses to allergic reactions. Diphenhydramine prevents histamine from combining with H1 histamine receptors. The latter are located in the following places:
- Central nervous system tissue
- Endothelium
- Smooth muscles
H1 histamine receptors determine elevated cell permeability and in combination with histamine they cause such reactions as:
- Allergic rhinitis symptoms
- Bronchial smooth muscle contraction
- Bronchoconstriction
- Itching
- Motion sickness
- Pain
- Separation of endothelial cells
- Vasodilatation
These receptors are also involved in sleep regulation. After administering Diphenhydramine, itching and other symptoms related to the allergic reaction are alleviated. The effect of this drug can be enhanced if corticosteroids are administered at the same time.
Recommended Daily Dosage
Diphenhydramine is purchasable in one of the following forms:
- Injectable Diphenhydramine with concentrations of 10mg/ml and 50mg/ml
- Tablets containing 12.5mg, 25mg or 50mg
- Suspension with a concentration of 12.5mg/ml
The recommended dosage for cats ranges between 0.25 and 0.5mg per pound, administered twice a day at equal intervals. As each cat reacts in a different manner to antihistamine medication, the duration of the treatment depends very much from one cat to another. The duration is also conditioned by the severity of the symptoms and by the appearance of any side effects. In order to make sure that the symptoms do not reappear, the cat should follow the entire treatment, even if the symptoms seem relieved.
Side Effects and Contraindications of Diphenhydramine
Some of the side effects that may be exhibited by your cat while receiving Diphenhydramine include:
- Decreased appetite
- Diarrhea
- Dry mouth
- Sedation
- Urinary retention
- Vomiting
If your cat suffers from any of the health conditions listed below, you should talk to the veterinarian about an alternative to Diphenhydramine:
- Allergic lung disease
- Angle-closure glaucoma
- Bladder-neck obstruction
- High blood pressure
- Hyperthyroidism
- Prostatic hypertrophy
- Pyelo-duodenal obstruction
- Seizure disorders
- Severe heart failure
In some situations, the veterinarian will recommend a substitute to this medication, while in others lowering the dosage will prevent any undesired reactions.
Drug Interactions and Overdose Risk
Diphenhydramine is known to interact with several drugs, and as a consequence of the simultaneous administration, it will enhance or decrease the effect of those medications. For example, it improves the effectiveness of epinephrine. On the other hand, heparin and warfarin are less effective when given at the same time with Diphenhydramine.
As Diphenhydramine is eliminated in breast milk, nursing cats should not receive such medication unless the benefits are far greater than the risks. Also, pregnant cats may deliver kittens with birth defects if high doses of Diphenhydramine are administered during the pregnancy.
An overdose with this type of antihistamine will cause central nervous system stimulation or depression. Other than that, the symptoms differ depending on whether a mild or a massive overdose was given. Some of the signs that indicate an overdose include:
- Clumsiness
- Coma
- death
- Respiratory depression
- Sedation
- Seizures
Please keep in mind that the most serious of the aforementioned symptoms are very seldom and appear only as a consequence of a massive overdose.