After a while into the churning, Vasuki was unable to get adjusted to the constant churning motion. This made him really sick in the stomach, and he started spewing the venom mildly at first. The poison was minimal initially, but as the churning progressed, Vasuki could not hold the poison or the haalahala within him and soon started spewing huge amounts.
The asuras being closest to the head felt its immediate effect, and one by one, they started falling sick. The intensity of the poison was so huge that even the deva gana and the entire bhoo-mandala started getting affected by it. The entire creation was at stake here.
Without knowing where to go, led by Lord Indra, the devas and asuras approached Lord Shiva.
Together they said- " Dear Lord, you are said to be the destroyer of all the sins, adharama, and impurities within and outside in this universe. Life derives its meaning cause of constant fear of destruction. Now we are in a state of panic due to the fast-spreading of poison generated by Vasuki. Since he resides in your neck, you must surely know how to contain it. Kindly guide us and ask Vasuki to take all the haalahala back so that the Amrit does not get contaminated by the poison and can continue churning the ocean. Lord, you are the only one who can get us out of this difficulty. Please help us."
Being kind at heart and easy to please, Lord Shiva smiled at the army of devas and asuras. He assured them of his protection, and he immediately appeared where the Samudra Manthan was taking place and, without hesitating for a second, started drinking the poison. The entire creation was stupified by the selfless act of Lord Shiva and did not know how to react.
After a while, life on the Earth gradually returned to normalcy. Due to the poison's intensity, Lord Shiva's body started turning blue, and he started losing consciousness after drinking a considerable amount of poison. Everybody started being scared for the life of the Lord. Goddess Parvathi, Shiva's consort, was the first to react. With her divine power, she gathered all the poison in his body into his throat and held it there with her fingers for a long-time till Lord Shiva consumed the last drop of the haalahal.
Once he had consumed all the poison, goddess Parvathi tied a snake running around its lord - Vasuki worried for his health around Shiva's neck to hold the Vish in its place. The poison was localized, and only Shivas neck turned blue without any other side-effects.
Henceforth, due to his blue neck - he came to be known as Neela Kanta, and since he holds poison in his neck, he is also called Vishakanta.
Vasuki was very ashamed of his behaviour and apologized to both Lord Shiva and Goddess Parvathi. Both of them forgave him since he had no control over the incident. Vasuki then ordered the snake around Shiva's neck to remain so until he could return and take the position to guard the Lord's life till eternity. The snake humbly accepted the master's order.
The asuras were furious with the devas as they had a huge casualty compared to the devas. Their murderous rage scared Indra. He managed to escape Bali' wrath by pointing out that it was the Asuras who has insisted on being near the head as opposed to the position they were given in the beginning.
The asuras were in no mood to listen to the voice of reason. Indra then sent his well-learned healers to treat the affected by removing the hazardous poison from their bodies. After all the affected asuras and devas were healed, everyone was scared to get back to churning.
Vasuki then said - " Lord Shiva has drunk even the last drop of poison that resided in my body. Also, now that I have gotten used to churning motion, I assure you I won't produce any more poison. So all of you are safe to resume the churning."
Thus assured by Vasuki and having averted the danger of being poisoned, both sides took to their positions. Before they could start, Vishnu told them that now the worst was over, they will only be bestowed with invaluable Ratnas, and due to Shiva and Vishnu's contribution, it should be split four ways.
Both devas and asuras acknowledged the contributions of the Lords's and agreed to the condition. With renewed vigour and the sole intent of gaining power and attaining immortality by consuming the Amrit, they continued the Amrit Manthan.
PART 1: Samudra Manthanam Story From Vishnu Purana & Shiv Purana
PART 2: Preparations for the Ksheerasagara Manthan or Churning of the Ocean
PART 4: Samudra Manthan 14 Ratnas List Items
PART 5: Lord Vishnu’s Mohini Avatar