...There was a king reigned in the East: There, when kings will sit to feast, They get their fill before they think With poisoned meat and poisoned drink. He gathered all that springs to birth 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. - A. E. Housman
Actually Mithradates was face with a rebellion and attempted to poison himself. To no avail, due to his preparations, so he was either (depending on the source) forced to ask his bodyguard to kill him with a sword or, weakened by age and the sub-lethal dose of poison he'd been able to take, unable to commit suicide and slain by his foes. According to memory and Wikipedia, that is.