Note: I'm writing outside my field. Reader beware. (Oh, and please correct my mistakes).
I have no training whatsoever in medicine and, honestly, my approach has always been to outsource my opinions to the experts. If an MD told me I had disease X, I was happy to accept that I had disease X. As a result, until I got involved in the skeptical movement, I knew next to nothing about quackery: while I never took woo claims seriously and went to proper medical doctors, I couldn't tell you why, say, chiropractic was nonsense. A second consequence of relying on experts was that I never paid much attention to what average people said about medicine ("folk medicine"), I didn't much care and didn't trust such people to know what they were talking about. Now that I've been paying attention to medical questions more - actually listening when people talk, reading some medical blogs, looking stuff up occasionally, etc. - I've come to realize just how much utter nonsense circulates even among intelligent people. I have heard smart friends and acquaintances confidently repeat all five the medical myths listed below and I only subsequently found out they were in fact contradicted by science. One learns.
Myth 1: MSG is bad for you. I've heard this one more times than I can remember from numerous intelligent people. I only discovered this is a myth the other day when the New York Times ran an article on it. Subsequently, I found an FDA review of the evidence, which concluded that, for the vast majority of people, eating a normal dose of MSG has no established clinical effect. A quick PubMed search confirmed there doesn't seem to be negative clinical effects associated with this food additive.
Myth 2: you should drink 8 glasses of water. Everyone I've ever met who has been on a diet knows about - and believes - this one. And yet Heinz Valtin conducted a review of the relevant scientific literature and concluded that: "Not only is there no scientific evidence that we need to drink that much [water], but the recommendation could be harmful, both in precipitating potentially dangerous hyponatremia and exposure to pollutants and also in making many people feel guilty for not drinking enough."
Myth 3: sweetner causes cancer. Again, several people have repeated this myth in my presence (including people who I respect a lot) because I use sweetner regularly. There is certainly no way I'm going to be able to shoot this idea down better than Steven Novella, so check out his blog entry that argues sweetner - or at least aspartame - has no demonstrated negative clinical effect.
Myth 4: depression is caused by a "chemical imbalance" or a serotonin lesion. This claim is ubiquitous - whenever depression comes up in conversation, someone is bound to hold forth on "chemical imbalances in the brain". In fact, the cause of depression is currently unknown. Despite this, as Jeffrey Lacasse and Jonathan Leo have demonstrated in two studies (pdf), pharmaceutical companies perpetuate the myth in their advertisements and the media parrots them uncritically.
Myth 5: drinking megadoses of vitamin C cures illness. At least one person who reads this blog (you know who you are!) has tried to convince me this is true. It's not. The idea seems to have originated with Linus Pauling, the great American chemist and (double) Nobel Laurette. Unfortunately, as the always reliable Quackwatch documents in detail, later in his life Pauling branched out into medicine and promptly went off the rails. There is simply no reliable evidence that large doses of vitamins cures either the common cold or cancer.