It causes immediate closing of the eyes and a burning sensation, difficulty breathing, runny nose and coughing.
The effects generally last about 30-45 minutes.
While the spray itself isn't lethal, it can cause death in combination with medical conditions like asthma.
Pepper spray was used on a mob of youths during a brutal Dargaville attack on two Northland police officers on Christmas Day in 2012.
Both officers were hospitalised, despite having used pepper spray and a Taser to defend themselves.
A few months earlier, Northland police had to use pepper spray to subdue a violent man who punched a constable in the face, breaking his tooth.
Local police declined to comment on the Northland figures, referring questions to national headquarters.
A spokesman said pepper spray use fluctuated over time, and did not reflect any local trends.
"Despite these fluctuations, crime nationally has been steadily dropping in recent years, as has the number of assaults on police."
This was partly thanks to increased access to a range of tactical options, including pepper spray, and training.
"Any police response is ultimately dictated by the actions and behaviour of those we are confronted with at the time, with OC spray just one of a number of potential options police may use.
"First and foremost however, an officer's best weapon is communication, which is successful in resolving the majority of situations we encounter."
Between March 2010 and June 2013, force was used in 0.2 per cent of face-to-face encounters.
Pepper spray was used in 32 per cent of these incidents.
As well as trialling more powerful OC spray and Tasers, police have also expanded their arsenal of non-lethal options to include a "sponge round" which can be fired from up to 35m away.
Police received regular training on pepper spray use, which focused heavily on sound judgement and good decision making "to resolve situations using the least amount of force possible, while maximising safety and minimising any risk to our staff, the subject and the public".
Any use of force by police must be lawful, justified, reasonable and proportionate to the circumstances, the spokesman said.
In the mid-1990s, the US Department of Justice cited nearly 70 fatalities linked to pepper spray use.
However, New Zealand Police Association president Greg O'Connor said pepper spray was a safer alternative to physical force and hugely effective as a deterrent for offenders.
"Most people who have been pepper-sprayed don't really want to be pepper-sprayed again."
It was also one of the best ways to avoid injury, he said.
"Pepper spray reduces the risk to both [officer and offender] by negating the need to have that physical contact and grapple with people.
"So many injuries are caused by flailing arms and knees while attempting to subdue someone who's going through some sort of psychotic episode."
The stronger pepper spray trialled in the Bay of Plenty late last year would be effective on those offenders who "literally" didn't feel pain because of drugs and alcohol, Mr O'Connor said.
