I have seen it fail to work, in a training class. One of the women in the class simply did not respond to the stuff. The instructor sprayed her. She blinked and said, "That's it?"
With her consent, he sprayed her again -- a longer spray from a closer distance. She kind of half-sneezed, half-snorted, and shrugged.
He went and got a fresh can and tried again. She shrugged and they gave up.
About five minutes later, I watched her wander over to the water buckets, splash a single scant handful of water on her face, rub it with a towel, and walk off. No redness, no snot-strings, no watery eyes. Nothing.
Most people respond to it. Some don't. And -- based on that class -- I would say the difference isn't drug/alcohol intoxication, but simply some folks don't respond to it.
Doesn't make it bad stuff. If I weren't severely asthmatic, I would probably carry OC at least some of the time. But I would not bet my life on it.
The inimitable Tamara tells the story of the time she used pepper spray to defend herself at this link
. Simply put, the spray did not
do the entire job, but it did buy her the time she needed to put another defense in place. I'd say that's a success story, but it does underline the important lesson about OC use: if you carry pepper spray to defend yourself, you should always always always have a back up plan for what you will do if the spray does not work.
(Even if the plan is, "Run like a striped-butt ape and scream like a little girl.")
More here: www.corneredcat.com/Gear/pepperspray.aspx