What follows is Massad Ayoob's opinion of that advice. Ayoob served on the Concord, New Hampshire, police department long enough to rise from a patrolman to the rank of Captain and is now one of the premiere self-defense instructors in the country, specializing in firearms. The strong formulas are sold only to police, and watered-down stuff to civilians. Since the complaints about suspects' eye damage in the mid-Sixties, all formulas have been weakened across the board. The Mace-type sprays have proven to be much less effective than once believed.
Many people can take several streams of the stuff square in the face with no immediate ill effects.
It only works right off when it's sprayed in the eyes, which is a no-no. Even that won't work if your assailant wears glasses, or shades. Drunks are almost immune. The instructions say to spray at the chest, so he'll inhale the rising fumes. It just doesn't work that way.