This proposal would see possession - though not distribution - of small quantities of dangerous hard drugs (such as heroin) decriminalized, or at least changed from a felony to a misdemeanor. The legal threshold (again, for possession) will fall just below the amount that your average user will consume in one day.
At present any degree of possession can result in felony charges in the US. What is the result of this? Your incentives as a user align the most with the goal of not getting caught. Assuming the police don't have a warrant to search your house, the time when you're at most risk of getting caught is when you're making a trade with your dealer.
If, hypothetically, you bought a week's worth of heroin in one go, and you did this every week, you (a daily user) would have to make some 52 trips a year to your local drug den (or spot on the street where you try to make a discrete 3-second exchange; whatever). You will have to risk 1/7th as many trades through this method; while being caught with larger quantities may worsen your sentencing in the event that you are caught, the lesser risk of getting caught in the first place may offset this in the mind of the average user. I would add that the drug-addled brain is less capable of long-term planning; you may perceive that there's no difference between a 15 year and 30 year sentence, whereas getting caught in the first place is a more short-term problem you do have reason to care about.
On the other hand, let's say you the average user wake up tomorrow only to learn that the law has been changed. If you only buy small quantities at a time (again, only for your personal use), then there's nothing illegal about that.
What are you going to do? Hit up your dealer once a day, every day, to purchase exactly one day's worth. You won't be as sneaky about it either, since you yourself are at no risk of arrest (or at least imprisonment).
This is from the average user's standpoint. But for the average dealer, whose activities emphatically are still a felony offense, now he has to make trades with said weekly buyers seven times as often.
This should make it soooooo much easier for the police to catch dealers - or, the chokepoint in the drug trade that, if successfully cut off, would nip the entire drug epidemic in the bud.