The Controlled Drugs and Substances Act and the courts make huge distinctions between drug users and drug traffickers. From a sentencing perspective, drug trafficker punishments could be 100 times more severe than drug user punishments.
Being accused of "simple possession" of a drug will usually lead to diversion (out of the judicial system), a discharge (avoiding a criminal record), a suspended sentence (with no additional punishment) or a small fine. By comparison, being accused of trafficking (or possession for the purpose of trafficking) could land you in a federal penitentiary for many years, depending on the type and quantity of drug trafficked.
Because of this huge distinction between possession and trafficking, a strong defence to any possession for the purpose of trafficking charge is that the drugs - regardless of their quantity, packaging and surrounding circumstances - are solely for personal use.
This defence isn't going to work for you if there is strong evidence that you actually sold or gave drugs to someone else, like an undercover officer, because then your tiny amount of possession will transform itself into a not so tiny amount of trafficking. Practically speaking, there's no such thing as a huge amount of simple possession (you'll always be accused of possession for the purpose of trafficking if found with huge quantities of a drug), and there's no such thing as a tiny amount of trafficking (while multiple traffickings and trafficking in large quantities will net you more prison time, the starting point for even smaller amounts of trafficking in "serious" drugs can be quite significant jail time).
But if you're one of the many who are found to have drugs on their person, in their vehicle, or at their home or business, and there's no evidence that you trafficked, the "they're for personal use" defence might work for you. There are a few especially good features of this defence.
First, it's one your lawyer might be able to negotiate for you with the Federal Crown, without your having to take your chances with a judge or jury by running a trial. If the Crown becomes convinced that there may be no "reasonable prospect of conviction" against you on the possession for the purpose of trafficking charge, perhaps because the quantity of the drug is fairly low, or there aren't lots of other indicia surrounding its possession which they can point to in support of a trafficking conviction, then a deal may be possible. The Crown may prefer the certainly of a possession conviction - certain because you will plead guilty - in preference to the iffiness of running a PforP trial that could end in an acquittal on all charges.
Two, if you do need to run a trial, in order to prove the charge the Crown will be dependant on the qualification and opinion of an "expert" who will testify that such a quantity of such a drug, together with any other indicia of trafficking surrounding its seizure from you, inevitably leads to the conclusion in his or her expert opinion that your purpose for the possession must have been trafficking. The problem with experts from the Crown's perspective is that it can be tough to find ones who are truly qualified to demand the opinion that is being asked of them. Sometimes the Crown will be forced to pull any detective constable out of the drug squad who's been running drug investigations for a few months, and seek to convince the court he can be an "expert."
Some "experts" may have minimal knowledge of drug use, and the amount an addict might reasonably keep for personal use purposes. Yes, if you're found with one metric tonne of cocaine, the expert's job then might be easy. But many cases involve grey areas for quantities that might be trafficking, and might not be trafficking.
Likewise, the so-called hallmark "signs" of drug trafficking that experts will testify about have no science behind them. When I started prosecuting drug cases back in 1995, we argued that everyone with a cell phone and a moderate amount of drugs must be a dealer. Because really, what were cell phones good for other than for doing drug deals? If you had a cell phone and a pager, or two cell phones, we argued there could hardly be any doubt at all over the intent of the accused! Times change.
Thus your lawyer can challenge the qualifications of an expert, challenge the weight threshold at which point one can start making claims that it can't be for personal use, and challenge the significance ascribed to items found in your possession as being probative of trafficking.
Lastly, to be successful at a possession for the purpose trial you don't need to convince the court that there's a reasonable doubt about you being in possession of the drugs. You only need to raise a reasonable doubt about the purpose of that possession.