Len Saunders, an immigration lawyer with a practice in Blaine, Washington, is warning Canadians that if they tell border guards they intend to buy marijuana, they risk a permanent ban from entering the United States.

Marijuana can now be legally bought and sold in Washington under state law, but possession and consumption of marijuana is still illegal under U.S. federal law. Saunders says many Canadians aren't aware of that.

"It's not a grey area, it's very black and white. At the ports of entry its still considered a controlled substance," said Saunders.

Saunders says he gets about two to three cases every month of Canadians barred from entering the U.S. for admitting to past marijuana consumption, and he says he expects to see a huge increase in his business because of Washington's new pot laws.

If you are going shopping for marijuana, you can just say you are going shopping. You don't have to go into details - Len Saunders, lawyer in Blaine, Washington

"I can only imagine that people are going to be forthcoming, telling officers at the ports of entry, that they are coming down to smoke marijuana," said Saunders.

Saunders says an admission of past marijuana use or stating one's intent to buy or smoke marijuana in the future can get Canadians a perpetual ban from entering in the U.S.

"The next time you enter, you will need a waiver. Waivers cost $585 U.S.. They take 4 to 6 months to get approved," Saunders said.

The waivers also aren't permanent, and need to be renewed every one to five years, according to Saunders.

Saunders says he doesn't advise Canadians to lie at the border, but he does think they should watch what they say to avoid problems in the future.

"If you are going shopping for marijuana, you can just say you are going shopping. You don't have to go into details," he said.