Chief constable Peter Neyroud has warned that his officers will take a tough stance on cannabis in areas where there is antisocial behaviour.
The drug was downgraded to Class C on January 29, and the Thames Valley chief said those found with small quantities were unlikely to be arrested.
Mr Neyroud said:"It will have to be more than mere possession to trigger a court case."
But he added his officers would get tough if necessary.
He said the drug would be confiscated from anyone stopped and they could receive an on-the-spot caution. Their details would be taken and those given repeated warnings arrested.
Officers would be entitled to suspect someone found with cannabis might be involved in dealing cannabis or harder drugs, prompting a search and background check.
Mr Neyroud added: "It may be, having checked that person, that they have a significant previous record for crime or drug abuse. We still have the power of arrest and to prosecute and there are some serious penalties for dealing.
"What we are trying to stop is people who are dealing in all sorts of substances -- it might be today they have cannabis but tomorrow they have ecstasy or heroin.
"The strong message that we are increasingly putting across is that where people are coming to us with complaints of antisocial behaviour making their lives hell, we will look at that and use our powers."
Mr Neyroud said there was no evidence that taking cannabis led to 'acquisitive' crime, such as robbery and burglary, and that tackling Class A drugs -- which are linked to offences -- was a priority for the force.
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