Slobodan Praljak drank a small vial of liquid after his prison sentence was upheld.
A war crimes appeal hearing in The Hague was cut short dramatically when one defendant drank what he said was poison upon hearing the verdict.
Slobodan Praljak, 72, was one of six former Bosnian Croat political and military leaders up before the court.
He was sentenced to 20 years' imprisonment in 2013 for crimes in the city of Mostar.
On hearing that his sentence had been upheld, he told the judge, "I have taken poison".
The six were attending the final appeals judgment to be handed down by the UN's International Criminal Tribunal for the former Yugoslavia (ICTY).
Though allies against the Bosnian Serbs in the 1992-95 war, Bosnian Croats and Muslims also fought each other for a period of 11 months, with Mostar seeing some of the fiercest fighting.
'Don't take away the glass'
Praljak stood and raised his hand to his mouth, tipped his head back and appeared to swallow a glass of liquid.
Presiding judge Carmel Agius immediately suspended the proceedings and an ambulance was called.
"Okay," the judge said. "We suspend the... We suspend... Please, the curtains. Don't take away the glass that he used when he drank something."
Before the curtains were lowered, the courtroom could be seen in a state of confusion, the BBC's Anna Holligan reports from The Hague.
An ambulance could later be seen arriving outside the tribunal while a helicopter hovered above the scene.
Several emergency rescue workers also rushed into the building carrying equipment in backpacks.
More than an hour after the incident, a court guard told Reuters news agency said Praljak was still "being treated".
Crimes against Muslims
Praljak, the former commander of the main staff of the Bosnian Croat defence forces (HVO), was jailed for crimes against humanity.
Informed that soldiers were rounding up Muslims in Prozor in the summer of 1993, he had failed to make any serious efforts to stop the action, the UN war crimes tribunal found.
Image copyrightREUTERS
He had also failed to act on information that murders were being planned, as well as attacks on members of international organisations, and the destruction of the city's historic Old Bridge and mosques.
Those appearing with him included Jadranko Prlic, the former prime minister of the Bosnian Croats' breakaway statelet.
Set up by mandate of the UN Security Council in 1993, the ICTY is due to close when its mandate expires at the end of the year.
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