What dilates your eyes?

to let in more or less light

Its caused by the iris. The iris contains two muscles that control the siz of the pupil. The circular, or sphincter, muscle contracts in response to parasympathetic stimuli or excessive light on the...

Eyes dont dilate with all types of weed, only some do. Dilated eyes is not always a side effect after smoking marijuana.

Maybe a concussion? That can cause one eye to dilate. I'd get that checked out.

No -- rather the opposite. Most opiates, opioid and similar drugs constrict the pupil.