He was. Napoleon used to give away locks of his hair as "gifts" and upon his death, more locks of hair were taken.
Researchers contacted the people who had these samples of Napoleon's hair and the dates they were taken, and they analyzed these hair with a mass spectrometer. They found increasing levels of arsenic rising to fatal levels just prior to his death.
Doctors also examined his described symptoms which were recorded at the time, and all his symptoms were consistent with arsenic poisoning. Further evidence is had in the fact that when his body was moved after having been buried for quite some time, it was in perfect condition with no signs of decay, as if he had died the day before.
This is also a characteristic of heavy arsenic poisoning because when arsenic gets into the cells of human tissue it kills bacteria that tries to consume the tissue.
That was all covered in the book I read about Napoleon’s (possible) murder. I have also read that high arsenic levels were found in hair from lots of people from that time, and that the amount of arsenic in locks of Napoleon’s hair at the time of his death, while much higher than what we’d see in people today was not that unusual among people of the First Empire period. Again, I believe he may have been murdered, but I do not believe this can be conclusively proved.