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How do sniffer dogs spot cannabis?

In many parts of the world, the possession and use of cannabis remains prohibited, despite the unstoppable wave of legalisation in some jurisdictions. Where marijuana use is prohibited, users have to be discreet and inventive to evade the police. Law enforcement agencies, on the other hand, rely on a major ally that is extremely difficult to fool: sniffer dogs. So how is it that dogs can be so effective in detecting cannabis?

Why use sniffer dogs to detect cannabis?

For centuries humans have understood the potential of dogs. Their intelligence allows them to be trained for specific tasks and their highly developed sense of smell has made them the ideal companion for hunters. More recently, another profession has taken advantage of these two qualities. They are used by police officers to detect drug users, especially cannabis users.

While humans have about 6 million olfactory receptors, dogs have about 300 million in their nose. But thats not the only reason why sniffer dogs are so good at it. The area of their brain that processes odours is also 40 times more developed than in humans. This means that dogs have an extremely sensitive sense of smell, which enables them to not only detect a person who has consumed cannabis, but also anyone who has been in contact with weed. They do this all the more easily because the olfactory particles of cannabis are very persistent. The dog’s final asset in detecting cannabis is its selective sense of smell. In other words, dogs are able to differentiate between the molecules that reach their nose. Whereas humans perceive a cocktail of smells that are difficult to distinguish, dogs perceive the various ingredients in the cocktail. It is therefore illusory to hope to deceive the dog squad by masking the smell of cannabis with other stronger smells. Humans will fall for it, dogs will not.

This ability to smell cannabis obviously depends on many factors such as environmental conditions, the amount of weed, its storage etc. However, in favourable winds and in the open air, a dog is able to spot cannabis at a distance of about 200 metres. In normal conditions, however, he should be about 50 cm away from the weed.

Dog squads usually use Malinois shepherds. Their sense of smell is not necessarily better than that of other breeds of dogs, but Malinois are easy to find and easy to train. In cases where a highly developed sense of smell is required, police officers use Springer Spaniel dogs, which have the added advantage of being hunting dogs and therefore used to searching. The training of a sniffer dog takes about 6 months, after which it is ready for service. In order not to lose the skills acquired during its training, it will nevertheless have to continue regular training, either in the field or in a training centre.

In the presence of cannabis or other drugs, dogs are trained to sit or lie down. Some dogs retain a scratching reflex when they spot an illegal substance. Trainers ideally seek to inhibit these behaviours to prevent the dogs from causing damage.

The limitations of sniffer dogs

Sniffer dogs are a valuable ally for police forces. But they are not infallible.

Firstly, they are unable to detect cannabis that does not emit any odorous particles in their direction. While there are no terpene-free varieties of cannabis, there are materials that are completely impermeable to prevent odour molecules from reaching the dogs. Aluminium foil or glass, for example, do not allow any molecules to pass through that a dog could detect, unlike plastic which, contrary to what many people think, is porous.

Another limitation of sniffer dogs is that they cannot detect drugs that do not emit any odour, such as LSD, for example.

Last but not least, the behaviour of sniffer dogs is influenced by that of their handler. A 2011 study in the journal Animal Cognition reports that the behaviour and non-verbal cues of the handler have an impact on the dogs’ ability to detect the products they have been trained to detect. Eighteen teams of drug and/or explosive sniffing dogs and handlers were tested. They were tasked with finding traces of drugs (or explosives) on four different pieces of paper. In reality, none of them contained the marks: they were either odourless or impregnated with the smell of food that was supposed to stimulate the dog’s curiosity. The authors of the study told some of the handlers that two of the four papers contained traces of drugs. When the handlers were convinced that drugs were present, the dogs made many more errors than when they were not misled by the researchers. Hence their conclusion that “handler beliefs affect outcomes of scent detection dog deployments.”[1]

A deeper problem than we think

This study has angered many dog units, particularly in the USA, who have refused to participate in this type of research again. Others have recognised that the dogs’ behaviour can be affected by their own behaviour and some services have decided to re-train dog units to address these issues.

This is not a trivial issue. Indeed, police officers are humans who have a vision of the world that is obscured by many biases, notably racial ones, as denounced by the Black Lives Matter movement, not only in the United States but also in the rest of the world. American police officers are much more likely to consider a black or Hispanic person as a delinquent, a cannabis dealer, a thief or other and to control him. This racist behaviour is then reproduced by the dog. However, as sniffer dogs are seen as neutral ‘tools’ incapable of racist bias, the recognition of the problem is complex. In the end, sniffer dogs prolong the racist behaviour of their owners, while making it even more difficult to denounce and fight them.


[1] Lisa Lit, Julie B. Schweitzer, Anita M. Oberbaue, ”Handler beliefs affect scent detection dog outcomes”, Animal Cognition, 2011, 14(3): 387–394 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3078300/).

Cultivation information, and media is given for those of our clients who live in countries where cannabis cultivation is decriminalised or legal, or to those that operate within a licensed model. We encourage all readers to be aware of their local laws and to ensure they do not break them.

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