My Dad has MS and there is a Cannabis based drug for that which he was given. He hated it and stopped taking it.
SativeX (if you can get someone to prescribe it). Which is essentially just cannabis (they go to a lot of trouble wrapping it in jargon to hide the fact that all they do is grow cannabis, extract the THC/CBD in precise ratios and dissolve them in a solvent).
Sadly because most medicines that would be based around cannabinoids start (as with SativeX) the whole plant, there are problems with patenting them.
As the partner of an MS suffer who decided a long time ago that I'd rather break the law than see a loved one in pain, I know that it's totally possible to prepare cannabis to relieve pain (more the spacisity which causes the pain) without psychoactive effects ("getting high"). Which suggests there are different components at work.
20 years ago, we would have been activists and pushing for a change - as those 20 years older than us did in the 70s.
Now we just quietly grow our own, try and achieve the best quality of life possible, and keep our heads down.
The effects of cannabinoids on cancer are not observed by smoking, or using cannabis recreationally. There are various stages to isolate particular types (THC and CBD have many variants) and from what I've learned, the resultant substance is usually consumed as a suppository with no psychoactive effects.
People can believe what they like. But the fact that cannabis is illegal, and St. Johns Wort (or opium poppies) are not is a case of res ipsos loquitor as far as it goes.
One thing which is true in medical research, is that scientists are becoming more used to the idea that individuals react to diseases in individual ways - which is why treatment that is effective for one person has no effect on another. (I have a family member in the US with a PHd who has made this their career. Current thinking is that there's a genomic relationship between disease and cure.). There's also a lot of evidence to suggest that individuals have an inbuilt rhythm which can affect the rate of absorption of medicines.
If this family member is right, then the future really will see individually tailored treatment, based on a persons genetics.
(If reading that you remember that medieval medicine had the idea that cures were based on astrology and celestial rhythms, you'd probably best keep it to yourselves. Modern scientists really don't like the idea that they're just reinterpreting previous bullshit ....)
Returning to the PP ... if you ask around in all the right places, you'll find there are quite a few people who have tried SativeX and found it unpleasant (if it worked) that have managed to use regular cannabis with much better outcomes.
Before we all bow down and worship at the altar of science, I'd highly recommend people check out Ben Goldacre and his works. They are a very good look at quackery and charlatans, but also highlight how "proper science" isn't above using a few tricks to fool the punters. Cherry picked research being one.
Addressing the OPs assertion ... never attribute to malice that which can be explained by incompetence. I don't think there's any great conspiracy from the big pharmaceutical companies to keep us ill. However the bottom line is that when the board of GlaxoSmithKline have a decision to make, they will more often than not make the choice which is best for the board of GlaxoSmithKline, rather than Mrs. Miggins of Acacia Avenue who isn't really getting on with .
And since I had to dredge that up, I'll leave you with a ...