From Wikipedia on January 9, 2010:

The first known mention of the word ABRACADABRA was in the 2nd century AD in a poem called De Medicina Praecepta by Serenus Sammonicus, physician to the Roman emperor Caracalla, who prescribed that the sufferer from the disease wear an amulet containing the word written in the form of a triangle:[1]

A - B - R - A - C - A - D - A - B - R - A
A - B - R - A - C - A - D - A - B - R
A - B - R - A - C - A - D - A - B
A - B - R - A - C - A - D - A
A - B - R - A - C - A - D
A - B - R - A - C - A
A - B - R - A - C
A - B - R - A
A - B - R
A - B
A

This, he explained, diminishes the hold over the patient of the spirit of the disease. Other Roman emperors, including Geta and Alexander Severus, were followers of the medical teachings of Serenus Sammonicus and are likely to have used the incantation as well.