According to Philip Dwyer's relatively recent biography Citizen Emperor: Napoleon in Power 1799-1815 (2013), the poison used by Napoléon was "a mixture of opium, white hellebore and belladonna". Dwyer does not provide a source for this so one has to look at the primary sources.
The main ones are the memoirs of several men who were present when Napoléon tried to commit suicide in Fontainebleau on the night of 12-13 April 1814. The most precise and pathetic one is that of Armand de Caulaincourt, a close adviser and Grand Squire of the Emperor. In posthumous memoirs published in 1933, Caulaincourt claims that Napoléon asked for him at 3am. He was extremely weak, but still chatted with an increasingly anxious Caulaincourt, who left the room to find help, notably that of Alexandre-Urbain Yvan, Napoléon's surgeon.
The Emperor called me, reproached me for disturbing his last moments; he despaired, complained of the slow effect of the opium preparation he had taken.
Caulaincourt then repeats that the poison was opium. Note that he did not mention the nature of the poison in a previous series of memoirs collected by Charlotte de Sor and published in 1837. The final edition of Caulaincourt's memoirs was published in 1933: in a footnote, editor Jean Hanoteau concludes that the poison did not contain only opium but also "white hellebore and belladonna" (see below), which is probably the source of Dwyer's version.
Another witness is Baron Fain, Napoléon's personal secretary. Fain was actually there (he is mentioned in other testimonies) but he does not talk about himself in his memoir so his narrative is of the "Third-person limited point of view" type:
At the time of the retreat from Moscow, Napoleon had procured for himself, in case of accident, the means of avoiding falling alive into the hands of the enemy. His surgeon, Yvan, had given him a bag of opium, which he had worn around his neck throughout the entire period of danger. Since then, he had kept this bag in a secret part of his kit with great care. That night, the moment had seemed to have arrived for him to resort to this last resource. The valet who was sleeping behind his half-open door had heard him get up, had seen him dilute something in a glass of water, drink and go back to bed. Soon the pain had forced Napoleon to admit that his end was near. It was then that he called his most intimate servants. Yvan had also been called; but hearing what had just happened, and hearing Napoleon complain that the action of the poison was not quick enough, he had lost his head and had fled hastily from Fontainebleau.
However, Fain adds a footnote:
It was not only opium; it was a preparation indicated by Cabanis, the same one that Condorcet used to kill himself.
The question of the nature of the poison used by Condorcet is another can of worms, so I won't go in there...
Constant Wairy, Napoléon's first valet describes the container of the poison but not its nature:
The emperor had gone back to bed, but as I walked towards his bed I saw on the floor in front of the fireplace the remains of a black skin and taffeta bag, the same one I mentioned before. It was indeed the one he had worn round his neck since the Spanish campaign, and which I had kept for him with such care in the interval between one campaign and another. Ah! if I could have suspected what it contained! At that fatal moment the awful truth was suddenly revealed to me!
Louis-Estienne "Ali" Saint-Denis, Napoléon's second valet, does not describe it either.
The last testimony is that of Yvan's son, published in a magazine in 1845.
On the point of leaving for the too memorable campaign of Russia, Napoleon had a sort of prediction of the reverses which were to befall him, and he was seized by the fear of falling himself into the hands of his enemies. Accordingly, he sent for his favourite surgeon, Dr. Baron Yvan, and after telling him of his fears, he asked him if he could not have a poison prepared which would be active enough to kill promptly and without too much pain. He added that he would carry it with him at all times, in a pill, for use if fortune should reduce him to this extremity. My father wanted to make some observations, but the emperor ordered him in such an imperative tone to carry out his orders, that he was obliged to obey. Baron Yvan immediately sent for M. Rouyer, the pharmacist-major of the Emperor's household, and had a powder composed of belladonna and white hellebore prepared before him [so: no opium in this version]. The not very active composition of this poison is, as we can see, a consequence of the ideas that my father had when he dared to make observations to the Emperor. This preparation was placed in a pill and given to His Majesty. During the disastrous campaign, Napoleon lost this jewel, and when he returned to Paris, he again ordered his doctor to prepare the same dose of poison for him. This time the crown jeweller made a small case in which M. Rouyer put the compound which the Emperor was always to carry in his waistcoat pocket, and which was even more quickly lost than the pill.
In the son's narrative, the Emperor complains to Yvan (or jokes?) that the "poison he gave him had no effect", but the surgeon believes that Napoléon is just having a crise nerveuse, a nervous breakdown, as usual. But when Yvan hears other people (Caulaincourt etc.) discussing the poison, he completely freaks out, believing that he may have had indeed poisoned his sovereign, and he leaves Fontainebleau in a hurry (Napoléon won't forgive him for that). For Yvan's son, his father never believed that Napoléon poisoned himself.
So that's it for the available testimonies. Nothing is really certain. The poison Napoléon drank in April 1814 may have been different from the one he had carried on him before. He certainly behaved like he had tried to kill himself and all witnesses report that he complained more or less cryptically about the poison, or the "dose", not being strong enough. The poison may not have been very strong, but then people forced Napoléon to vomit relatively quickly, and he was fine in the morning. Wairy and Saint-Denis, the two valets, say that Yvan gave him a beverage that had some effect, but the other witnesses do not mention it. The symptoms described by Caulaincourt - spasms, vomiting, sweating, agitation - seem more compatible with hellebore and belladonna than with opium. Ultimately there is no way to ascertain what really happened.
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