Pet owners are usually cautious about giving human medications to cats and dogs-- and with good reason. According to the American Society for the Prevention of Cruelty to Animals (ASPCA), human medicines account for the vast majority of accidental poisonings in pets. Fortunately, diphenhydramine hydrochloride (trade name Benadryl) is one of the few over-the-counter remedies for humans that can be used safely in veterinary medicine. However, since well-meaning pet owners can easily overdose a small animal, it is best to use the drug in cats only if the compound is prescribed by a veterinarian.
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Uses in Cats
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Diphenhydramine (Benadryl) is not often used for allergies in cats; most veterinarians recommend corticosteroids and other medications as a first choice for allergic cats. However, diphenhydramine does successfully block the over-production of histamine, leading to near-instant relief for cats suffering from severe allergies. Diphenhydramine is most frequently used in cats as a treatment for motion sickness and vomiting, and it is also a common remedy for itching caused by insect bites and other forms of skin irritation.
Dosage
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Pet owners may find it extremely difficult to give a cat a proper dose of Benadryl, since human preparations are designed for much larger doses than cats can safely tolerate. Diphenhydramine pills are primarily available in 12.5 mg and 25 mg capsules, but most pet owners find it easier (and safer) to use liquid preparations, which can be measured with more precision. Liquid "baby" Benadryl usually comes in a 12.5 mg/ml suspension. Cats should be given no more than 1 mg of diphenhydramine per pound of body weight, no more than once every 12 hours. Since Benadryl has an overwhelmingly bitter taste to cats, many of them will drool profusely after taking the medication.
Considerations
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While most cats can tolerate small to moderate doses of Benadryl with few serious side effects, others may be extremely sensitive to the drug's effects. Veterinarian Barbara Forney warns that older cats are especially sensitive to the sedative properties associated with diphenhydramine, and they may require a smaller dose of the medication. She also notes that Benadryl should be used with caution in animals with "prostatic hypertrophy, bladder neck obstruction, severe heart failure, angle-closure glaucoma, pyelo-duodenal obstruction, hyperthyroidism, seizure disorders, hypertension, or allergic lung disease (COPD)" due to an increased risk of serious or life-threatening side-effects. Antihistamines can also interact with multiple veterinary drugs.
Signs of Overdose
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Like humans, cats can experience fatigue and clumsiness while taking diphenhydramine. This side-effect can be more noticeable if the cat has experienced a mild overdose, but it should pass within a few hours. In more severe overdose cases, the cat will experience seizures and difficulty breathing. Without treatment, severe overdoses can lead to coma and death, so it is critical that owners seek prompt treatment for the cat's condition. The ASPCA also gives real-time consultations for pet owners through the Animal Poison Control Center, which can be reached at (888) 426-4435. Immediate medical attention can stop an overdose from becoming lethal.
When to See a Vet
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If a cat shows symptoms of allergies for more than seven days, it may require evaluation from a veterinarian. The allergy-like symptoms may actually indicate a respiratory infection, which could require treatment with antibiotics or decongestants. Cats with chronic allergies may also need specialized care to prevent allergy-related complications like ear infections and asthma. Some pets may need a specialized diet or nutritional supplements to minimize their allergic symptoms. Concerned pet owners should see a vet before self-diagnosing or self-treating any condition in an animal.
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