d-limonene
Limonene is a terpene compound in citrus peel. Cats lack sufficient glucuronidase enzymes to metabolize it, causing seborrhea, salivation, vomiting, and liver damage. Often appears as 'natural fragrance,' 'citrus extract,' or 'grapefruit seed extract' — a hidden ingredient cats especially need to avoid.
Other common names: limonene, citrus oil extract, citrus extract, orange oil, lemon oil, grapefruit seed extract, citrus peel extract, 檸檬烯, 柑橘精油, 柑橘萃取, 葡萄柚籽萃取, d-リモネン
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Sources: ASPCA Animal Poison Control Center, FDA Center for Veterinary Medicine, veterinary literature. For reference only, not medical advice. If poisoning is suspected, seek veterinary care immediately.