[Heather]

Heather asked the Naked Scientists:

I recently discovered your podcast and am trying to catch up and listen to all
of the previous downloads!

In a recent show you talked about why our eyes dilate in the dark.  Does
the explication that it has to do with a chemical process also apply to why
people who take hallucinogenic drugs have dilated pupils?

What do you think?

[RD]

"Street" drugs can cause large dilated pupils and tiny constricted "pinpoint" pupils...

Signs of [opiate] intoxication are decreased respiratory rate and pinpoint pupils.

http://www.emedicine.com/EMERG/topic20.htm

[Bored chemist]

The drugs mimic or block the chemicals that the body uses as messengers to tell the eye's muscles to contract or relax.
There's more about those messengers here
http://en.wikipedia.org/wiki/Neurotransmitters