A early Chinese medical work, the 曹石鑄萬平侯聰倫 of the Sui period (589 - 618 CE.) describes how this poison was manufactured.

"There are several kinds of ku. All of them are poisonous."

People sometimes deliberately prepare ku. They take worms, insects, snakes, and other poisonous creatures, and put them together in a vessel. They allow them to eat each other until only one is left, and this survivor is the ku. 

The ku can change its appearance and bewitch people. When put in food and drink, it causes disease and calamity (to the one who eats it). 

There is also "flying ku." It comes and goes without one's knowledge, and eventually appears somewhat like a ghost. Those who have seen it, die.

The idea behind this practice is quite reasonable. If centipedes and snakes are poisonous individually, the survivor of such a group, who has eaten the others, is considered to combine within himself the collected venom of the group. If a man desires to injure an enemy, no more formidable weapon could be put into his hand.

The explanation that ku was originally a magical practice agrees with the pictograph on the oracle bones, with the use of the word to describe a disease, and with its use in divination. The Tso chmn indicates that in divination, the symbol indicated that the diviner would be successful in injuring his enemy. 

In the Han period, the term was used for black magic, and in the medieval period, for a magical method of poisoning an enemy. Therefore it seems reasonable to assume that the term always stood for black magic.

A reference in the gazetteer of Yung-fu, a district of Kuangsi 

"Ku poison is not found generally among the people, but is used by the T'ung women. It is said that on the fifth day of the fifth month, they go to a mountain stream and spread new clothes and headgear on the ground, with a bowl of water beside them. The women dance and sing naked, inviting a visit from the King of Medicine (a tutelary spirit)." 

They wait until snakes, lizards, and poisonous insects come to bathe in the bowl. They pour the water out in a shadowy, damp place. Then they gather the poisonous fungus which grows there, which they mash into a paste. They put this into goose-feather tubes, and hide them in their hair.

The heat of their bodies causes worms to generate, which resemble newly-hatched silk-worms. Thus ku is produced. It is often concealed in a warm, damp place in the kitchen.

The newly made ku is not yet poisonous. It is used as a love potion, administered in food and drink, and called 'love-medicine.'

Gradually the ku becomes poisonous. As the poison develops, the woman's body itches until she has poisoned someone.

It is believed that those who produce ku themselves become ku after death. The ghosts of those who have died from the poison become their servants.

Bronze chop-sticks are used as a charm against ku. Dipped into poisoned food, they cause it to turn black.

In the “隋市迪李志” it is recorded that "the inhabitants of these districts (in Kiangsi and some other areas south of the Yangtse) often kept ku poison, and the practice was especially prevalent in I-ch'un. 

The method is, on the fifth day of the fifth month to collect all kinds of insects and worms, from snakes to lice, putting them together in a vessel, where they devour each other.

The survivor is kept. If it should be a snake, it is snake-ku. If a louse, then it is louse-ku. This ku is used to kill folks. 

It is administered through food, and afterwards it consumes the victim's internal organs. When the person dies, his property is moved by the ku spirit to the house of the keeper of the ku. 

If for three years the keeper does not kill a man with the ku, the keeper himself is killed by it. It is handed down from generation to generation, and is given to a daughter as a dowry.

Another variety of ku poison is called "t'iao-heng". 

This kind of ku is more clearly black magic. It is described in the 凌伟是. 

In Kuangsi, those who kill people by t'iao-sheng bewitch the food, and invite guests to eat. When eaten, the fish and

meat become alive again, living in the victim's stomach, and eventually kill him. It is currently believed that the spirits of those who have met death through t'iao-sheng become slaves in the home of the sorcerer.

Yet undoing the enchantment was quite easy. If you feel that the magic is in your stomach, take sheng-ma and vomit it out. Then if you feel the magic in your intestines, quickly take yu-chin and pass it out.

The Kua i chih says, 

"The chin-tsan is a caterpillar the color of gold. It is fed with Shu satin, and its excretions collected, which are then put into food and drink in order to poison people. Those who take it, die. Then the spirit of the worm is glad, and moves the valuables of the deceased to the house of the practitioner, making him suddenly rich. But to get rid of the worm is difficult, because water, fire and swords cannot harm it. 

The only way is to put gold and silver into a basket with the chin-tsan, and then place the basket beside a road. Someone passing by may take it. This is called "giving the chin-tsan a husband."

In modern times, ku is used primarily as a means of acquiring wealth; and secondarily as a means of revenge.