With the understanding that when the nectar was generated from the churning they would share it equally, the demigods and the demons brought Vasuki to be used as the rope for the churning rod. By the expert arrangement of the Supreme Personality of Godhead, the demons held the snake near the mouth, whereas the demigods held the tail of the great snake. Then, with great endeavor, they began pulling the snake in both directions. Because the churning rod, Mandara Mountain, was very heavy and was not held by any support in the water, it sank into the ocean, and in this way the prowess of both the demons and the demigods was vanquished. The Supreme Personality of Godhead then appeared in the form of a tortoise and supported Mandara Mountain on His back. Then the churning resumed with great force. As a result of the churning, a huge amount of poison was produced. The prajapatis, seeing no one else to save them, approached Lord Shiva and offered him prayers full of truth. Lord Shiva is called Asutosa because he is very pleased if one is a devotee. Therefore he easily agreed to drink all the poison generated by the churning. The goddess Durga, Bhavani, the wife of Lord Shiva, was not at all disturbed when Lord Shiva agreed to drink the poison, for she knew Lord Shiva’s prowess. Indeed, she expressed her pleasure at this agreement. Then Lord Shiva gathered the devastating poison, which was everywhere. He took it in his hand and drank it. After he drank the poison, his neck became bluish. A small quantity of the poison dropped from his hands to the ground, and it is because of this poison that there are poisonous snakes, scorpions, toxic plants and other poisonous things in this world.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Eighth Canto, Chapter 7 – Introduction