Human beings, myself included, have this fascinating flaw where we hear information, assume it’s true, and go around repeating it. The end result is that a lot of garbage is floating around as facts.

• You know that crazy lady who sued McDonald’s and successfully won because her coffee was too hot? It’s surely either urban legend, or was a ridiculous, frivolous lawsuit, right? Nope! It was a real court case, Liebeck v. McDonald’s Restaurants, and the facts suggest it’s actually a very legitimate lawsuit. More here.
• Poinsettias aren’t particularly dangerous to pets. They are mildly toxic, but [the ASPCA says] they are “generally over-rated in toxicity.” Don’t feed them to your pets, but they’re not the enormous danger the news, for some reason, makes them out to be.
• Diet soda (specifically, aspartame) doesn’t cause cancer. The American Cancer Society calls talk of this “rumors” which “continue to circulate the Internet.” They also address talk about how aspartame, when digested, includes methanol (toxic!) as a by-product, pointing out that, for example, fruit juice produces ten times as much. They go on to debunk, in detail, the myth that diet soda / aspartame causes cancer.
• Vaccines don’t cause autism. This myth came about in 1998, when British “former surgeon” Andrew Wakefield published a study in The Lancet arguing that the MMR vaccine was linked to autism. Trust in the vaccine plummeted. As Wakefield’s study came under fire for undisclosed conflicts of interest, The Lancet called his study—which it had published—”entirely flawed.” They went on to entirely retract the article, and ultimately deemed it a fraud, showing that Wakefield had forged data. His article spurred others to investigate his claims; subsequent, credible studies have not reached the same conclusion. Autism Speaks, Inc., an advocacy group for those affected by autism, says of subsequent studies: “These studies have not found a link between vaccines and autism,” and that they “strongly encourage” that parents vaccinate their children.
• MSG, which everyone knows as that thing that used to be in Chinese food until it was banned for being carcinogenic or something, is actually recognized by the FDA as being safe. Knee-jerk reactions caused it to practically disappear from use, but scientists have never found any actual correlation. (Interestingly, though, it seems like there is a strong placebo effect among those who think they are allergic to MSG.) You can even buy MSG on Amazon, where it seems to have pretty good reviews.
• High-fructose corn syrup is in a similar position. It’s pretty much vilified, and studies have found all sorts of health issues associated with it. But, here’s the thing that’s key: those health issues are the same as with normal sugar. Excessive intake of either is bad. There’s currently little in the way of evidence that substituting HFCS for sugar causes health issues.
• MDMA (ecstasy, the illegal drug) doesn’t cause holes in your brain, and has generally had its neurotoxicity massively overstated. Using ecstasy is still a bad idea, and there are plenty of legitimate health issues with using it. However, many of the “facts” that we hear about it are egregiously wrong. For one, the holes in the brain thing is thought to have come about through a false-color CAT scan, which was actually showing very minor, temporary reduction in blood flow to regions in the brain; the regions with reduced blood flow were colored differently, and someone interpreted them as “holes”. A number of other studies about MDMA’s neurotoxicity have been similarly flawed. One was retracted when it was found out that the study administered meth to lab rats instead of ecstasy. A number of other studies have compared lifelong users of many drugs (“polydrug users”), including ecstasy, to people who have never used drugs. It is unsurprising that the former group has more neurological problems; the studies entirely fail to show that ecstasy plays a role, though.

I’m not actually advocating many of these things. All things equal, I prefer sugar over HFCS, even though there’s not strong scientific support for this. I have never tried ecstasy, and have no desire to do so; there are many legitimate health issues. I don’t drink a lot of diet soda because I don’t care for the taste. I’m not sure that anything includes MSG anymore, but I’m not buying it as a supplement and pouring it on my food. And don’t feed your dogs poinsettias. But it drives me insane when people go parroting these things as “facts,” when they range from mostly untrue to patently false.