Loathe is a verb. Loath (also spelled loth) is an adjective.
Loathe means to hate. Loath means reluctant or unwilling:
I loathe hypocrisy. (verb)
The politician was loath to admit that he had taken the bribe. (adjective)
A mistake with the verb loathe is to use it as if it were an adjective:
Incorrect: He’s loathe to invest a lot of time pursuing another profession.—Abbotsford News (Canadian).
Correct : He’s loath to invest a lot of time pursuing another profession.
Incorrect: [Fogarty] is loathe to criticise the project.—The (London) Telegraph
Correct : [Fogarty] is loath to criticise the project.
Incorrect: He is loathe to criticize the mayor on WEP.—The Baltimore Chronicle
Correct : He is loath to criticize the mayor on WEP.
Loathe and loath are not pronounced the same.
Despite its many irregularities, English spelling does have rules. One of the rules is that (most of the time) adding an e to th at the end of a word indicates that the th is pronounced with the voiced sound.
Note: Th is voiced in the word this. Th is unvoiced in the word thin.
Here are three common pairs that illustrate the rule:
Here are six more words that conform to this rule and are pronounced with the voiced th:
Two exceptions are smooth and bequeath, which are spelled without a final e but are pronounced with a voiced th.
Note: Merriam-Webster gives the unvoiced pronunciation of bequeath first and the voiced pronunciation second. The Oxford English Dictionary shows only the voiced pronunciation.
Given the number of pronunciation guides like M-W and Howjsay that seem to condone the merger, attempting to preserve the distinctions between the spellings and pronunciations of adjective loath and verb loathe may be futile.
Nevertheless, I think that keeping the adjective and the verb separate is worth the effort, as do my two style guides of choice:
The Chicago Manual of Style
To loathe something is to detest it or to regard it with disgust: “I loathe tabloid television.” Someone who is loath is reluctant: “Tracy seems loath to admit mistakes.”
The Associated Press Style Book
loath (adjective); loathe (verb). Note the difference: “She is loath to leave.” “He loathes bureaucracy.”
Related post: Loath and Loathe