It is extremely common for a word to have a vowel added onto the end if it does not necessarily resolve easily.
I think you will probably find, that it was called Absinthe long before the compound that causes the hallucinations was found, and was subsequently named Absinthium, in reference to the most common place where it was found.
However, pedantry is always welcomed.
(, Thu 11 Dec 2008, 0:14, Reply)