Even though the pot laws are being proposed to change in Canada and marijuana is legal in 25 US states, you cannot cross the US border if you have a drug charge no matter how small and how long ago.
The Americans continue to be very strict about drug charges and can even deny you entry to the US if you admit to smoking pot.
If you have never been denied entry to the US and you have a drug charge, you can obtain a pardon or record suspension to get it removed. Once your pardon is granted, you should be safe to travel to the US without worry.
If you have been denied entry to the US due to a drug charge or any other criminal offence for that matter, you will need a US Entry Waiver (I-192, I-194). A US Entry Waiver is a complicated and time consuming application that will allow you entry to the US even after you have been denied entry.
Here is a recent article titled “Canada looks to end US travel ban for residents who admit to marijuana use”: