From the many-venomed earth;
First a little, thence to more,
He sampled all her killing store;
And easy, smiling, seasoned sound,
Sate the king when healths went round.
They put arsenic in his meat
And stared aghast to watch him eat;
They poured strychnine in his cup
And shook to see him drink it up:
They shook, they stared as white's their shirt:
Them it was their poison hurt.
-I tell the tale that I heard told.
Mithridates, he died old.
Mithridates died at the age of 57, which isn’t terribly ancient but is pretty good for a king of the ancient East, considering how many people attempted to kill him during his reign.  He was finally done in when Pompey the Great defeated his armies and he committed suicide to keep from being taken prisoner by the Romans.  There are two versions of his initially-botched suicide. 
 Appian: 
    Mithridates then took out some poison that he always carried next to his sword, and mixed it. There two of his daughters, who were still girls growing up together, named Mithridates and Nysa, who had been betrothed to the kings of Egypt and of Cyprus, asked him to let them have some of the poison first, and insisted strenuously and prevented him from drinking it until they had taken some and swallowed it. The drug took effect on them at once; but upon Mithridates, although he walked around rapidly to hasten its action, it had no effect, because he had accustomed himself to other drugs by continually trying them as a means of protection against poisoners. These are still called the Mithridatic drugs. Seeing a certain Bituitus there, an officer of the Gauls, he said to him, “I have profited much from your right arm against my enemies. I shall profit from it most of all if you will kill me, and save from the danger of being led in a Roman triumph one who has been an autocrat so many years, and the ruler of so great a kingdom, but who is now unable to die by poison because, like a fool, he has fortified himself against the poison of others. Although I have kept watch and ward against all the poisons that one takes with his food, I have not provided against that domestic poison, always the most dangerous to kings, the treachery of army, children, and friends.” Bituitus, thus appealed to, rendered the king the service that he desired. 
 Cassius Dio: 
 Mithridates had tried to make away with himself, and after first removing his wives and remaining children by poison, he had swallowed all that was left; yet neither by that means nor by the sword was he able to perish by his own hands. For the poison, although deadly, did not prevail over him, since he had inured his constitution to it, taking precautionary antidotes in large doses every day; and the force of the sword blow was lessened on account of the weakness of his hand, caused by his age and present misfortunes, and as a result of taking the poison, whatever it was. When, therefore, he failed to take his life through his own efforts and seemed to linger beyond the proper time, those whom he had sent against his son fell upon him and hastened his end with their swords and spears. Thus Mithridates, who had experienced the most varied and remarkable fortune, had not even an ordinary end to his life. For he desired to die, albeit unwillingly, and though eager to kill himself was unable to do so; but partly by poison and partly by the sword he was at once self-slain and murdered by his foes. 

Terence this is stupid stuff