Because drug offences are largely "victimless" crimes, meaning there's usually no one in whose interests it is to report them to the police, authorities rely heavily on intrusive investigative techniques to discover these offences. These intrusive techniques are also needed to obtain samples of the alleged "drugs" in order to test that they aren't in fact drywall compound or icing sugar.
While warrants and privacy interests existed prior to 1982, the adoption and constitutional entrenchment in 1982 of s. 8 of the Canadian Charter of Rights and Freedoms (the protection against unreasonable search and seizure), combined with s. 24 of the Charter (authorizing a court to exclude evidence obtained in violation of the Charter) placed a new emphasis of the protection of privacy interests of Canadians against state intrusion. The most fundamental way to protect privacy is to require the state to obtain a warrant from an independent judicial official prior to conducting a search.
Like a lot of legal things, when a warrant is and isn't required is not black and white, but rather occupies a realm of shades of grey. However, there is a clear pecking order of privacy interests where the greater the privacy, the more likely a warrant will be needed. In any situation where drugs are discovered through a search leading to criminal charges, it's possible to argue as part of a pre-trial Charter motion that a warrant should have been obtained prior to conducting the search, and that therefore the drugs should be excluded from evidence at trial. The usual consequence of no drugs in evidence will be a collapse of the prosecution's case.
The most common situations of drugs being discovered through a police search are: (1) in a vehicle; (2) on a person or in something a person is carrying; (3) in a building. A warrant might be required to search any of those places.
Warrant needed for a vehicle
For vehicles, police often claim that their search is "incident to arrest" and therefore a warrant isn't needed. But the law limits the scope of such searches to only relate to the reasons for the arrest. Thus police can't conduct a traffic stop, issue a speeding ticket for which no arrest would occur, and then poke around in a vehicle on a fishing expedition looking for drugs.
Police will often claim a vehicle is being searched pursuant to "consent" from the occupant. While I completely understand the psychological pressure you might be under to say yes to the police question "do you mind if I take a peek in your trunk," just say no. Either they have authority, or they don't. Saying yes won't earn you any brownie points.
Warrant needed for a person
Searches of persons or the things they're carrying are also often justified under the "incident to arrest" banner. They key here to legality is there must be a valid arrest to start with. If not, a warrant may be required to search things being carried, like a gym bag. Usually one wouldn't obtain a warrant to search a person's clothing, but personal searches involving bodily integrity (like taking blood samples or x-rays) would almost always require a warrant.
Warrant needed for a building
Other than the human body, buildings or portions of buildings which the public usually don't have any access to tend to have the highest expectation of privacy. This is particularly so with residences. Even if the police are already in a building for another legitimate purpose, they can't just go poking around looking for evidence - they need to get a warrant. In extreme situations, they should be "freezing" the scene and getting a warrant, rather than later claiming exigent circumstances didn't permit obtaining a warrant.
So arguing that a warrant was needed to search wherever drugs or related evidence was located remains a key part of any strong drug charge defence. You should consult a lawyer with experience in search warrants to obtain advice about whether such a defence could work for you.
