I'll tackle two different, but closely related topics at once here.
Firstly, I have noticed that among all the "major" and "classic" conspiracy theories out there, there are a bunch of other "minor" conspiracy theories, advocated by some people.
One of these is that the FDA (Food and Drug Administration, an agency of the United States Department of Health and Human services that regulates and supervises, among other things, medications and medical treatments) has a primarily profit-driven agenda, and are using their power to shut down any forms of alternative competing medicine that would be more effective and cheaper than the established forms of medicine.
Especially miracle cancer cures seem to be one of the favorite subjects of this. There are many websites, "documentaries" and videos out there about several such "miracle cures" and how the FDA is either ignoring them or outright actively trying to shut them down and stop people from using them.
This mentality is so American-centric that it gives me nausea. These people talk as if the United States were the only country in the world with any kind of medical research and practice. (Or, what's worse, alternatively they seem to imply that the FDA has power over the entire world, not just the US.)
The whole idea is ridiculous. If someone found a working cancer cure, medical science would be interested. Even if (and that's a big if) the FDA had a selfish agenda and was indeed fighting against these alternatives, surely some other country would investigate the claims further and perform some tests? Are these people saying that all the medical establishments in the entire world are into the same conspiracy, or independently driving the exact same agenda? That's just ridiculous.
Secondly, some people's attitude towards cancer and its treatments is just outright irrational. Sadly, in many cases this attitude is either greatly fueled, or in fact triggered by those conspiracy theories.
I have conversed with several such people. They usually have some heartbreaking story about a grandmother, an aunt or other family member (usually their own, sometimes a close friend's) who had cancer and went through a debilitating course of heavy chemotherapy which in the end didn't work, making only this relative's life a living hell for as long as he/she had left of it. They have the firm conviction that if they ever get cancer, they will refuse any medical treatment, and instead seek for alternative cures, no matter what. They will not go through the same hell as their relative. They call chemotherapy "poison".
These people are idiots.
Their mentality seems to be that there's approximately one type of cancer (it's just "cancer", that's it), modern medicine has no idea how to cure it, and they are simply using their patients as lab rats for their experiments, while ignoring alternatives.
One of the biggest mistakes they are making is engaging in confirmation bias. What they are completely ignoring are the numerous cases where chemotherapy and/or the other forms of treatment do work. Cancer treatment has advanced in giant leaps in the past decades, and the live expectance of cancer patients has increased significantly. Even complete cures have increased significantly. In many cases the side-effects of the therapy are only minor.
Of course it doesn't always work. There are many different types of cancer. They differ in type, location, size, severity and probably a myriad of other factors. Each type of cancer requires its own specialized treatment, and no treatment can cure them all. Even when using chemotherapy, the treatment has to be specialized for the exact type and severity of the cancer.
Unfortunately sometimes it just doesn't work. Some cancers are too severe, too advanced or otherwise too difficult for the treatment to work.
However, consider the altenrative: Not doing anything. (Or trying to seek for "alternative cures", which usually amounts to the same thing.) You are not much better off: You will wither, you will suffer, you will become just a husk of your former self, and you will die in horrible agonizing pain (or so drugged you will not be aware of anything) as your organs deteriorate and shut down one after another.
Granted, aggressive chemotherapy can make this suffering even worse, but at least you tried. After all, there is a good chance that the chemo, or any of the other forms of treatment, will actually work, and give you another 10 or 20 years of life, which you wouldn't have if you just stubborningly refuse treatment because you think that the doctors are "poisoning" you.
Refusing treatment would be especially foolish in the milder cases, with the easier types of cancer that have a much higher rate of success, with only mild or even no adverse side-effects.
If someone who could have been cured refused treatment because of this, I blame the conspiracy theorists. The blood is in their hands.