What is appropriate usage of medicines that are intoxicating?

I think the key word in the fifth precept isn't "intoxicating" but Pamadatthana i.e.

Pamāda
carelessness, negligence, indolence, remissness

See also other topics on this site which describe pamada:

• What is the purpose of the 5th precept?
• How do different traditions define "intoxicant"?

Apparently, in Thailand doctors make fewer prescriptions for cancer pain than you might expect -- I don't know whether that's because of Buddhism though.

I think I read once that pain itself can, if it's intense enough, cause "heedlessness" -- perhaps a Buddhist would prescribe a medicine-which-causes-heedlessness to the extent it's necessary to alleviate pain-which-causes-greater-heedlessness.

Buddhist monks are allowed to take medicines which they "need". I think it's another monk who might decide whether they "need" something (or at least decide whether they need to see a doctor), but also monks aren't allowed to act as medical doctors for laypeople, so there's not much answer to your question from that source.

A very general answer -- a way to maybe answer start to answer it yourself - might be to relate it to remorse: i.e. if you feel "remorse" after doing something perhaps that's a sign that it would be better to steer clear of doing that.

See also Experiencing physical pain

My impression is medical cannabis does not have an intoxicating effect.

Regardless, the precept on intoxicants refers to heedlessness. If a person has a serious medical condition and must use ordinary intoxicating cannabis for relief then it is expected they would be morally mature enough for the cannabis to not lead to heedlessness.

As long as it is for true medical purpose as a treatment when there are no other similarly effective alternatives, and one is not using possible medical benefits as an justification but consuming it for toxication, then it is fine.

General sprescribed medication as a treatment and given but a doctor is fine.

If one seeks toxicating meditations of the toxicating benefits then it is not OK.

Let me ask you this, can you enter samadhi when you're high? How easy is it to maintain mindfulness when you've smoked a bowl? Is your awareness as sharp? Can you establish atapi or sampajanna? I know when I used to smoke, getting rid of anything that could be construed as ardent and clearly comprehending was kinda the point.

One thing that I think gets lost in regards to the fifth precept is this blunting of perception. When we reduce this training rule to only those conditions that cause heedlessness, we ignore the larger way this precepts figures into our training. I mean, think about it. Are all of those fermented substances listed by the precept really the only things that cause intoxication? Of course not. There are plenty of other psychoactive compounds and not all of them are chemically derived. Porn, biased news coverage, our phones, and countless other vices remove us from full engagement with experience. They cut us off from the moment. There is only so much mental real estate we're afforded. When we crap it up with these mental intoxicants, there's no room left for the path.

Of course, this doesn't mean we should beat ourselves up. If a life saving medicine makes you a little woozy (e.g. chemotherapy), by all means go for it. You aren't much use as a Buddha if you're dead. But if the choice is between popping a couple of Benadryl and going all balloon head or having a running nose, sit down on the cushion and let your nose drip down your chin. I think you'll find that samadhi is a rather impressive panacea. More importantly, our egos hate it when we suffer. It's an affront to everything it stands for. Embrace these minor irritations and club it over the head.