The entire yew bush, except the aril (the red flesh of the berry covering the seed), is toxic due to a group of chemicals called taxine alkaloids. Their cardiotoxicity is well known and act via calcium and sodium channel antagonism, causing an increase in cytoplasmic calcium currents of the myocardial cells. The seeds contains the highest concentrations of these alkaloids. If any leaves or seeds of the plant are ingested, urgent medical advice is recommended, as well as observation for at least 6 hours after the point of ingestion. The most cardiotoxic taxine is Taxine B followed by Taxine A; Taxine B also happens to be the most common alkaloid in the Taxus species.

Yew poisonings are relatively common in both domestic and wild animals who consume the plant accidentally. The taxine alkaloids are absorbed quickly from the intestine and in high enough quantities can cause death due to general cardiac failure, cardiac arrest, or respiratory failure. Taxines are also absorbed efficiently via the skin and Taxus species should thus be handled with care and preferably with gloves. Taxus baccata leaves contain approximately 5 mg of taxines per 1g of leaves. The estimated (i.e. not by any means a fact) lethal dose (LDmin) of Taxus baccata leaves is 3.0-6.5 mg/kg body weight for humans There is currently no known antidotes for yew poisoning, but drugs such as atropine have been used to treat the symptoms. Taxine remains in the plant all year, with maximal concentrations appearing during the winter. Dried yew plant material retains its toxicity for several months and even increases its toxicity as the water is removed, fallen leaves are also toxic.