All Narcissus species contain the alkaloid poison lycorine, mostly in the bulb but also in the leaves. Members of the monocot subfamily Amaryllidoideae present a unique type of alkaloids, the norbelladine alkaloids, which are 4-methylcatechol derivatives combined with tyrosine. They are responsible for the poisonous properties of a number of the species. Over 200 different chemical structures of these compounds are known, of which 79 or more are known from Narcissus alone. The toxic effects of ingesting Narcissus products for both humans and animals (such as cattle, goats, pigs, and cats) have long been recognised and they have been used in suicide attempts. Ingestion of N. pseudonarcissus or N. jonquilla is followed by salivation, acute abdominal pains, nausea, vomiting, and diarrhea, then neurological and cardiac events, including trembling, convulsions, and paralysis. Death may result if large quantities are consumed. The toxicity of Narcissus varies with species, N. poeticus being more toxic than N. pseudonarcissus, for instance. The distribution of toxins within the plant also varies, for instance, there is a five times higher concentration of alkaloid in the stem of N. papyraceus than in the bulb, making it dangerous to herbivores more likely to consume the stem than the bulb, and is part of the plant's defence mechanisms. The distribution of alkaloids within tissues may also reflect defence against parasites. The bulbs can also be toxic to other nearby plants, including roses, rice, and cabbages, inhibiting growth. For instance placing cut flowers in a vase alongside other flowers shortens the life of the latter.

Many cases of poisoning or death have occurred when narcissi bulbs have been mistaken for leeks or onions and cooked and eaten.