The berries of various species contain Theobromine, a compound similar to caffeine. In very small doses, theobromine and other caffeines only mildly stimulate the nervous system. However larger amounts can cause dizziness, stomach pain, nausea, diarrhoea, elevated pulse rate, and low blood pressure, as well as drowsiness.
Berries attract birds that eat them after the frosts have reduced toxicity. However, if household pets ingest Holly, they are very liable to be poisoned, and it is a very good idea to keep holly decorations out of reach of pets and/or children