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I wonder if her husband's nickname is "Red."

A lot less than you would imagine.  Depending the person's size and overall health status, there's a good chance one little bottle would be enough.  Visine is pretty innocuous only because it's in eyedrop form -- it keeps the drug in the place where you want it to work.  Very little gets into the bloodstream to cause side effects.  Drugs delivered like this (eyedrops, topical creams, etc.) often contain way more of a drug than would be put into an oral formulation because of this reduced chance of side effects.  Tetrahydrozoline (the drug in visine), when taken orally, works on the nervous system and can decrease breathing, lower blood pressure, and cause seizures and coma.  Ingesting more than a few drops would definitely land someone in the hospital, even if it didn't kill him.  

I used to work at a bar that was frequented by some ladies of the night, and they often used Visine as knock-out drops.  A few drips in his drink, and their "date" would wake up to find his wallet and jewelery missing.  The girls would let the guys keep both kidneys, but I think that's only because they lacked the medical skill to get one out.

The lesson here is to be wary of hookers who treat their red, irritated eyes.

I learned a lot from the comments on this post.