Tuesday, March 10, 2009

Seeing Red!

The Legend:  An allergic reaction to marijuana is what makes your eyes red when you smoke it.

Status:  False


To really appreciate this particular legend you have to imagine a group of young people sitting around in someone's basement, in a circle, smoking pot.  They become intoxicated and look across the circle at each other, noticing that some of them have bloodshot eyes. 

"Whoa, dude, you should see your eyes, they're like SO bloodshot!" 
"I know, dude, so are yours!" 
"You know, I heard it's caused by an allergic reaction to the pot. Your eyes are red because you're both allergic, we all are!" 
"No way!" 
Riotous laughter ensues.

If you are a stoner reading this and are offended by the characterization of pot smokers saying lame things, it will be hard for you to mount a defense. A brief read through some online forums and chat rooms looking for answers explaining why marijuana causes bloodshot eyes yielded the following fascinating variations:
  • Marijuana increases your heart rate and that means your blood pumps more vigorously which causes red eyes.
  • Your brain calls for more blood because it heats up.
  • It's the coughing and laughing that leads to the bloodshot eyes, not the pot.
  • Marijuana dries out your eyes like a decongestant.
  • It's because you're forgetting to blink.
  • When you smoke marijuana your tear ducts relax and produce lubricants at a slower rate.
  • Weed gets into your bloodstream and when it goes through the blood vessels in your eyes it irritates them which turns them red.
  • Smoke makes them dry and stings them.
  • "Because pot messes with the nerves of your bran and that triggers your eyes."

What really happens is not nearly as interesting as these theories. Marijuana has a mild vasodilating effect. That means that it relaxes the smooth muscles within the walls of the blood vessel. This widens the blood vessel making it more visible. As a result, the eyes appear bloodshot.

You may have heard the rumor that marijuana is used to treat glaucoma, a condition where the fluid pressure in the eye increases and causes damage to the optic nerve. This is true, there has been some experimental activity in this area, (Tomida, et. al. 2006). When blood vessels relax and expand one of the results is reduced vascular pressure.