Loath, loathe, loth
Loathe is a verb meaning to dislike greatly. Loath is an adjective meaning unwilling or reluctant. Loth is a variant of loath and serves no purpose of its own.
Examples
Loathe is commonly misused in place of loath, even in editorially fastidious publications. For example, these writers use loathe in place of the adjective loath:
He was loathe, for example, to pay for his son to go to Cambridge … [Boston Globe]
Wenger would be loathe to take any undue risks with the player … [Telegraph]
I went to a church and sang carols, which I am loathe to confess because of the cliches of academics having a left-of-center bent and admitting to nothing remotely religious. [The MIT Tech]
These writers use loathe correctly:
I loathe Valentine’s Day. [The Trentonian]
But as much as I loathe technology, I need to know when writing letters will finally become obsolete. [The Maine Campus]
And these writers use loath correctly (i.e., as an adjective):
I had traditionally been loath to join a board game unless coerced … [The Brown Daily Herald]
Yet conservatives are loath to credit Harry Truman, who devised the original policy of containment … [BusinessWeek]
In speech, loath is usually pronounced with a harder -th sound (as in bath), and loathe has a softer one (as in bathe and breathe).