In the past, I have been approached by several friends and colleagues asking if they should buy pepper spray for themselves or their significant others. When anyone asks me for safety or self defense advice I always counter with the question, “What are your concerns?” This is partly because there is no cookie-cutter answer to any safety questions and partly, because I am trying to assess whether the goal is safety or piece of mind (perceived safety). In the case of pepper spray, typically piece of mind.
Pepper spray can be a very useful tool when it comes to self defense. Pepper spray is made from the same compound that makes chilli peppers hot, Oleoresin Capsicum (OC), but it’s much more potent. Some of the effects would include a burning sensation accompanied with involuntary eye closure and inflammation, a well as, excessive mucus discharge from the nasal cavity and the feeling of not being able to breathe.  Although the effects sound bad, all are typically temporary and subside in as little as 24 hours, but may last as long as a week.  If not used correctly permanent damage can occur, such as spraying directly into the eyes at a distance closer than 1 meter. This is called the hydraulic needle effect.  It is caused by the amount of pressure behind a direct stream of liquid at close distances. The particles of the active ingredient, OC, literally penetrate the soft tissues of the eye. 
As you can imagine, being sprayed isn’t exactly pleasant. With that being said, pepper spray is not going to work on everyone. Individuals “who are highly aggressive, agitated, intoxicated, or suffering from mental illness may have altered perception of and response to pain, and consequently may not be affected by –or may even become enraged after being –sprayed.”  Speaking from experience, despite the effects of the spray, it is really nothing more than being very uncomfortable.
I generally do not recommend buying pepper spray. As I mentioned earlier, most people buy pepper spray for piece of mind, not for actual self defense. This is how it typically goes down, a husband buys his wife pepper spray because she parks in an unattended parking garage. At first the wife has the pepper spray in her hand or at a minimum knows exactly where it is in her purse. Time passes and still no need for the spray, complacency sets in and the spray is somewhere in the purse, if in her possession at all. More time passes. There is still no need for the spray. It’s possible that she has forgotten about it all together until an incident happens when she needs it. If she remembers the spray, she has no idea where it is. By whatever chance she does manage to get it out and use it, cross contamination is likely. Feeling the effects of the spray for the first time, while you are being attacked can not have a good outcome. The last thing you want is to be incapacitated ,by your own doing, in a attack.
I will, however, recommend pepper spray under the following conditions:
1) Feel the effects yourself – doesn’t necessarily need to be sprayed but feel how it affects your breathing, skin, and eyes, while continuing to fight.
2) Practice properly using it. Be sure you know where it is and practice pulling it out and spraying at a assailant. Inert spray can be purchased for training. Also, practice moving to a new location or running and calling 911. You want to move because the assailant is going to attack the last position he saw you before being sprayed.
Overall, pepper spray can be a useful tool for self-defense, if you know how to properly use it and understand how you will be respond if you come in contact with it yourself. If you aren’t willing to feel the effects and fight through it, your basically buying piece of mind until an attack occurs.
Here’s a good example of the wrong way to use pepper spray.
1. Individual Defensive Tactics, Pepper Spray, http://defensive-products.idtac.com/pepper_spray.htm
2. Investigative Ophthalmology &Visual Science, Effects of Oleoresin Capsicum Pepper Spray on Human Corneal Morphology and Sensitivity,http://www.iovs.org/content/41/8/2138.full#aff-3, Feb 15, 2000
3. The Howard League for Penal Reform, Fact Sheet 18 – The Use of Oleoresin Capsicum (OC) – Pepper Spray, http://www.howardleague.org.nz/?p=422, Sept 30, 2001
5. C.Gregory Smith MD, MPH and Woodhall Stopford MD, MSPH, Health Hazards of Pepper Spray, http://duketox.mc.duke.edu/pepper%20spray.pdf, 1999