Re: Why does marijuana make your eyes bloodshot?

I could not find any studies that directly answered your question.  
However, based upon evidence obtained from studies on other tissues and 
after conferring with Dr. Paul McGuire (Department of Cell Biology and 
Physiology, University of New Mexico), I believe that the answer is that 
marijuana (specifically, cannabinoids) produce vasodilation (increased 
caliber of blood vessels).  The paper written by C. Hillard (reference 1) 
listed below reviews evidence that marijuana produces vasodilation.  In 
many cases, there is evidence that this effect may be mediated by what are 
termed CB1 receptors.  Although this paper does not discuss effects of 
cannabinoids on vasuclature in the eye, Porcella et al. (see reference 2) 
demonstrate that there are significant numbers of CB1 receptors in the eye. 
 So, it is possible that marijuana dilates the blood vessels in the eye by 
binding to CB1 receptors.

Your question may also be related to the interest of a number of scientists 
/ pharmaceutical companies in developing and using synthetic cannabinoids 
for the treatment of glaucoma.  The antiglaucoma properties of marijuana is 
the result of its ability to lower intraocular pressure (which may, in 
part, be due to its ability to produce vasodilation).  The Porcella 
paper lists several references related to this topic.


REFERENCES

1   Hillard, C.J.  (2000)  J. Pharmacol. Exp. Ther. 294, 27-32.

2   Porcella, A., Maxia, C., Gessa, G.L., and Pani, L.  (2000)  Eur. J. 
Neurosci. 12, 1123-1127.