The Latter-day Saints subscribe to a revelation dubbed the "Word of Wisdom" which outlines a code of health which Saints in good standing are expected to live. It's found in Doctrine and Covenants 89. Generally it prohibits use of "herbs" (drugs) such as tobacco for the body, but recommends it for treating cattle.
However, the LDS Church is pretty clear that drugs which are prescribed by a competent medical professional (a doctor) are allowed within the recommendations of the doctor. (This has traditionally meant things like morphine, sleeping pills, etc.)
When there is sickness or disease and healing is desired, there are a few recommended courses of action, such as obtaining a blessing with faith under the hands of priesthood brethren. Just as important to Latter-day Saints, however, is seeking care using the best medical knowledge available.
I've seen this question online before, but haven't found any answers that are definitive or official: How does the Word of Wisdom or LDS doctrine in general apply to medicinal marijuana (prescribed by a doctor)? Is it prohibited entirely, are there only some permissible cases, or is it A-OK? (Assume it is legal in the politick; Mormons believe in following the law of the land.)
Again, I'm posting here because I'm sure it'll attract higher-quality answers than I've seen elsewhere which are more speculation than citing official or declarative sources.