Yes, hydrangeas (Hydrangea macrophylla and other species within the Hydrangea genus) are considered toxic to both dogs and cats. The primary toxic compound found in all parts of the plant—including the leaves, flowers, and buds—is a cyanogenic glycoside known as hydrangin. When ingested and subsequently metabolized, this compound can break down to produce cyanide. However, it is crucial to understand that the concentration of this toxin is typically quite low in hydrangeas. The plant produces these compounds as a natural defense mechanism against herbivores, microbes, and fungi. The level of toxicity can vary depending on the specific species of hydrangea, the part of the plant consumed, and the stage of its growth cycle.
The cyanogenic glycosides in the plant tissue are not harmful in their stored form. Toxicity occurs when the plant material is chewed and consumed, allowing the glycoside to come into contact with enzymes also present in the plant. This enzymatic reaction hydrolyzes the glycoside, releasing hydrogen cyanide (HCN). Cyanide is a potent cellular toxin that inhibits cytochrome c oxidase, a key enzyme in the mitochondrial electron transport chain. This effectively prevents cells from utilizing oxygen, leading to cellular hypoxia. For a pet, the onset of clinical signs usually occurs within a few hours of ingestion. The most common symptoms of hydrangea poisoning include gastrointestinal upset, such as vomiting and diarrhea, which may sometimes be bloody. Lethargy, depression, and abdominal pain are also frequently observed. In very rare cases of massive ingestion, more severe signs related to cyanide toxicity could potentially occur, such as increased heart rate, panting, and incoordination.
From a botanical perspective, the risk of severe poisoning from a hydrangea is generally low to moderate. The plant's low palatability and the relatively small amount of toxin present mean that a pet would need to consume a significant quantity of plant material to risk life-threatening cyanide poisoning. Most cases involve mild to moderate gastrointestinal distress. The bright and showy flowers, which are actually modified leaves called bracts that surround the smaller, fertile flowers, are often the most attractive part of the plant to curious pets. It is also worth noting that the concentration of cyanogenic glycosides can be higher in young, growing leaves and buds compared to mature tissues. Therefore, a puppy or kitten chewing on a new growth branch may be exposed to a slightly higher dose of the toxin.
If you suspect your dog or cat has ingested any part of a hydrangea plant, the first step is to remove any remaining plant material from their mouth and identify the plant. Contact your veterinarian or a pet poison helpline immediately for advice. Be prepared to provide information such as the type of plant, the amount estimated to have been eaten, and the time of ingestion. You should not induce vomiting unless specifically instructed to do so by a veterinary professional. Treatment is typically supportive and may include monitoring, anti-emetic medications to control vomiting, and fluid therapy to prevent dehydration and help flush the system. In the vast majority of cases, with prompt veterinary attention, pets make a full recovery.
