Citrus fruits — lemons, limes, oranges, grapefruits — can upset a dog's stomach and irritate skin.
The fruit flesh is usually only mildly upsetting, but peels, leaves, stems and seeds contain higher concentrations of citric acid and essential oils.
Citric acid and the essential oils limonene and linalool irritate the gastrointestinal tract and, in larger doses, depress the central nervous system. Concentrated extracts (citrus essential oils) are far more toxic than the fruit itself.
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Small amounts of fruit flesh are unlikely to cause more than mild stomach upset. Avoid peels, pith and concentrated extracts entirely.