"Don't drink out of plastic bottles, that's why Christina Applegate got breast cancer!"

Right, because it's possible to pinpoint the exact cause--and the fact that her mother had it is just a coincidence. Of course, originally this circulated saying that was why Sheryl Crow had breast cancer.

"Light colored eyes let in more light, so people with pale eyes get more headaches."

Ummm ... light goes in through the pupil, not the iris. The iris is a muscle. Light doesn't filter through it. Critical thinking, people, and basic optical anatomy. Now, it's entirely reasonable to say that people with light eyes/hair/skin are more sensitive to light, but that's a different statement altogether. I think it's hilarious that a doctor told somebody this.

"OMG, don't use tampons! They make your period longer because they hold the blood in!"

I think some dude made this up. Any woman who's used a tampon knows they absorb the discharge, it's not like you're sticking a cork up your snatch to hold it all in!

"Eating meat is hard on your body."

*facepalm* Yeah, it's not like we've got enzymes to break down the proteins or anything, right? Humans certainly didn't develop eating a variety of foods, largely protein sources. I hate it when people say things like this. It might take longer to digest beef than, say, an apple--but that doesn't mean it's taxing for your system! This sort of thing, to me, smacks of rabid vegan/vegetarian attempts to convince people that eating meat is some strange modern adaptation, as if we never ate meat until the industrial revolution and so it's an unnatural state of being. But driving a smug-inducing hybrid car is totally natural!