Diphenhydramine Side Effects in Cats

Diphenhydramine is a medication that is formulated for use in humans only but is provided to cats as an extra-label medicine for use in controlling allergies. This drug is an antihistamine, or a medicine that prevents the cat's body from producing and receiving histamines, chemicals that occur naturally in the system when in the presence of an allergen or other sensitive substance. Diphenhydramine is available without a prescription, but it can have some potentially serious side effects in certain cats, so it is best to familiarize yourself with these before giving your pet this drug. As with any other medication, it is also sensible to work closely with a veterinarian to monitor your pet's health before, during and after completion of the drug regimen.


One of the most common side effects of diphenhydramine is excessive drooling. This is not typically a serious concern or a potential threat to your cat's wellbeing, but it can nevertheless be a discomfort to the animal. Diphenhydramine is typically given as an oral medication, and the bitter taste can sometimes trigger excessive salivation in cats. The result is that many animals begin to drool uncontrollably. Generally, this only lasts for a short time after ingestion of the medicine. However, if the drooling persists for more than an hour after your pet has taken diphenhydramine, consult with a veterinarian.


In some cases, diphenhydramine can cause indigestion in cats. Diarrhea and vomitting are not uncommon, as is a loss of appetite. Monitor your pet closely for signs of indigestion, as prolonged or continued indigestion can lead to negative health effects. Generally, however, these side effects are not serious.


Diphenhydramine has the tendency to make cats unusually drowsy or lethargic. Do not be surprised if your pet appears to be somewhat disoriented or sedated after taking the medicine. If your pet collapses or is immobile, however, call a veterinarian immediately.

Serious Side Effects

Diphenhydramine can react negatively with several other medications if present simultaneously in your cat's body. Many different types of tranquilizers, epinephrin and other medicines will produce a variety of serious and potentially harmful reactions if mixed with diphenhydramine. For this reason, it is crucial that you inform your veterinarian of all other medicines that your cat has ingested before beginning a treatment program involving diphenhydramine. Also, be careful that your cat does not have access to any of these medicines.

Cats with certain preexisting medical conditions may suffer from serious adverse reactions to diphenhydramine as well. If your pet has liver damage or kidney disease, he should not take diphenhydramine. As with other medications, it is crucial that you inform your veterinarian of any preexisting health conditions that your pet has before beginning a treatment program.

Compared with many other medications, diphenhydramine is relatively safe. A small percentage of cats may experience an allergic reaction to the medicine itself, but this is a minimal concern. If you have any additional questions about diphenhydramine, or if your cat appears to suffer from any unusual reactions to the drug, notify a veterinarian.