[1] The plant is a large, herbaceous, climbing perennial, with the stem woody at the base, up to 5 centimetres (2 in) in diameter; it has a habit like the scarlet runner, and attains a height of about 15 m (50 ft).

The flowers, appearing in axillary peduncles, are large, about 25 mm (1 in) long, grouped in pendulous, fascicled racemes pale-pink or purplish, and heavily veined.

Unfortunately, the antagonism between physostigmine and atropine is not perfect, and Sir Thomas Richard Fraser demonstrated that sometimes the action of the two drugs is summated and death results sooner than from either alone: namely, atropine will save life if three and a half times the fatal dose of physostigmine has been taken, but will hasten the end if four or more times the fatal dose has been ingested.

Although thus highly poisonous, the bean has nothing in external aspect, taste or smell to distinguish it from any harmless leguminous seed, and disastrous effects have resulted from its being incautiously left in the way of children.

The beans were first introduced into Britain in the year 1840; but the plant was not accurately described until 1861, and its physiological effects were investigated in 1863 by Sir Thomas Richard Fraser.