Vitamin C therapy
There are a great many people in the world who believe that vitamin C can be a treatment for lots of conditions. Linus Pauling - a two-time Nobel Prize winner, no less - was perhaps the major proponent of this treatment. Originally he thought it could treat cardiovascular disease and the common cold, but he soon moved into the classic position of claiming he could treat cancer with it. His work was widely criticised for its methodology by many other researchers, with claims of improper selection of trial participants being the worst faux pas: the patients on the therapy were actually less sick than the control group in one study.
Despite the refutation of this and other claims the treatment continues to be pushed to to this day. Part of the problem is that there appears to be loads of research and lab work on the subject, but much of it is pre-clinical research - animal or in vitro studies - which doesn't carry as much weight. Additionally, there is plenty of research that Vitamin C does nothing at all to cancer; or that it could both assist with the effectiveness of chemotherapy, or make the chemo drugs less effective. Meta-analysis of the various claims and studies seems to show that overall, there is no effect or they highlight issues with the trials, such as low sample size or ineffective blinding or grouping.
Media coverage of the various pieces of research doesn't help, with screeching headlines about how we should eat more vitamin C in response to these studies - and the issues highlighted with studies do not always make it into the final text of these articles. There is plenty of research into this area still going on. This doesn't, of course, mean that you shouldn't make sure that you aren't getting the RDA of vitamin C for other reasons. But you should treat claims that it is a wonder cure for many ailments, including cancer, with some skepticism - at least until the claims can be proved with a high degree of confidence.