"Stop poisoning children!"
Sometimes a treatment with unpleasant side effects is the only effective treatment
Imagine there was someone who believed that cancer wasn't real. That adults with cancer really just had a chemotherapy fetish, and children with cancer were being coerced into it by abusive parents.
They could point to the admittedly horrible side effects of chemotherapy and the fact that it's effectively a form of poison and ask dramatically what kind of monster could possibly subject a child to this. They could discuss how common the early symptoms of cancer are and how often they have a different cause, proclaiming that they had a cough once and had a narrow escape from being poisoned by oncologists. And in their worldview, this would all be completely logical and proportionate.
Once they've talked themselves into this belief, there's no way to talk them out of it. Anyone who tries to explain that yes, chemotherapy can be brutal, but the alternative is letting children suffer and potentially die, has marked themselves as a monster who defends poisoning children. Any medical authorities that lay out the process for diagnosing and treating cancer are simply demonstrating how effectively they've been captured by the cult.
The rest of us understand that doctors don't typically recommend drastic treatments on a whim. They're far from infallible, but they're much more likely to err in the opposite direction and dismiss serious conditions as anxiety. We understand too that sometimes a drastic treatment with brutal side effects is the only appropriate one for a similarly drastic condition. But these are fairly nuanced messages compared to a stark slogan like "stop poisoning children".
Fortunately, enough people have had a brush with cancer, or are close to someone who has, that they can dismiss a conspiracy theory like this without wasting any time on it. Now, if it was a less common diagnosis, especially one that had already been made subject to various moral panics, then we might really have a problem.