For the verb meaning to make sanitary, sanitize is the usual spelling in the U.S. and Canada, and sanitise is preferred everywhere else. The spelling difference extends to most derivatives, including sanitized/sanitised, sanitizing/sanitising, and sanitizer/sanitiser.
Sanitize is the original spelling, and until recently it prevailed throughout the English-speaking world. Over the past couple of decades, however, many British writers and publishers have begun using sanitise, and the spelling has also caught on elsewhere outside North America. This form is so new that it is still unlisted in most dictionaries. Even the unabridged Oxford English Dictionary is yet to recognize it.
Sanitize is part of the large class of verbs that traditionally took -ize endings, even in British English, because of their origins in Greek and Latin. These are in contrast to the verbs that take -ise endings, which in general come directly from French sources. In American English, there is a longstanding convention of changing most -ise endings to -ize, and this has given rise to the mistaken view that -ise is simply British and -ize American. Over the last several decades, in an effort to differentiate British English from American, many British publishers have begun giving -ise endings even to words that have always been spelled -ize. This has also occurred with realise (traditionally realize) and organise (traditionally organize), for example, and there are many others.