Bill Karwin commented on this:

There doesn't seem to be any syntax for DROP CONSTRAINT in 4.0 or even 5.0.

There's DROP FOREIGN KEY, which might do what you want.

See syntax at http://dev.mysql.com/doc/refman/4.1/en/alter-table.html

http://forums.mysql.com/read.php?10,85032,85036#msg-85036