Claudius died on 13th October 54. For a Roman, he was an old man: he was born in 10 BC. Rumours abounded that he had been poisoned. Although there are many instances of such accusations in early imperial history, in this instance, there is some evidence that his death was hurried on.
The detail in the accounts is impressive. In Dio (61.34), we are told that Agrippina consulted Locusta, a renowned poisoner [how did one become a renowned poisoner?]. The first attempt failed because Claudius drank too much and the effects of the poison were lessened. He took precautions against poisoning, but eventually Agrippina gave him a specially prepared mushroom.
Tacitus has a slightly different story. Claudius was poisoned by mushrooms, but seemed to be getting better. His doctor administered another poison (Tacitus, Annales 12.67) and with more success. Tacitus, clearly aware that accusations of poisoning would be read sceptically, said that all authorities had (a version of) the story.
Nero supposedly referred to mushrooms as the food of the gods, because Claudius became a god through eating mushrooms (Suetonius, Nero 33). It is notable that he felt able to joke about poisoning the man who was his adoptive father.
The imperial family mourned him and made him a god, even as they made fun of him (Seneca, Apocolocyntosis).
Why remove Claudius?
The decision to poison Claudius requires explanation. Agrippina could have just waited until Claudius succumbed to illness or some naturak cause. The time-sensitive issue was Britannicus. He was born in 51 and was thus on the cusp of assuming the toga of manhood. He had every reason to suspect his father’s wife and not feel well-disposed to his brother/brother-in-law. If Britannicus was to be raised up, the imperial house would be divided and there would be a choice between Britannicus and Nero. If Britannicus were to emerge as emperor, Agrippina and Nero would be in some danger.
In retrospect, it looks as though Agrippina was preparing the move for a long period. The key for Agrippina was to secure the loyalty of the praetorians. Those thought to favour Britannicus were removed and her man was put in place. Narcissus fell ill and sent away from Rome. With him out of the way, there was no one to watch over Claudius or to advise Britannicus. Everything was ready for a smooth transition, other than the neccessary requirement for an imperial death.
But that could be managed.
After Claudius was dead, Narcissus was killed.
Agrippina in Power Claudius Conclusions Accession of Nero