More than 2,900 police officers in Essex will continue to use CS gas spray for self defence, despite concerns by the Police Complaints Authority.
Essex Police Chief Constable, David Stevens, said this week the spray was a "worthwhile addition" to equipment as it allowed violent criminals to be restrained without using batons.
But the annual report of the Police Complaints Authority states: "Despite its authorisation for use on the public, the authority is concerned there has been no research into the possible interaction of CS spray with anti-pyschotic medication.
"We know that such drugs already produce adverse effects which include changes in heart rate, blurred vision, allergic rashes, rapid breathing, dizziness and changes in blood pressure.
"Some of these are also suspected side effects of exposure to CS spray .
"There may therefore be an increased risk where CS spray is used on someone taking such medication."
The spray was used 500 times in the county last year with 18 complaints, three of which have now been resolved, six withdrawn and nine are still being investigated.
The PCA says nationally there have been 500 complaints about the spray, the second highest area of concern.
Just over half the complaints analysed related to the spray being used in an enclosed space and a number followed incidents where the spray was squirted into cars and vans.
The PCA also discovered that 44 per cent of complaints were about the spray not being used in self defence.
One of the "key factors" were allegations that sprays had been used from a distance of less than three feet when the recommended range is three to 15 feet.
The Chief Constable said: "What we are finding is that if someone is being too violent and too dangerous for an officer to handle simply by holding them, CS spray is used at that stage to protect the officer before resorting to the baton."
Since the introduction of CS spray and the baton, attacks on Essex Police officers have reduced substantially.
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