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The Canadian government proposed last month that legal pot be sold in plain packaging that displays a slew of warnings and other information, including an expiry date. But does weed actually expire?

It depends on the cannabis you buy and how you store it.

As cannabis expert and Quebec brand manager of marijuana giant Canopy Growth, Adam Greenblatt, explains, dried pot flowers have more of a “best before” date, ensuring freshness and quality.

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“Dried flower generally has quite a long shelf life, but after about a year, it starts to lose its smell,” Greenblatt said. “The THC (tetrahydrocannabinol) doesn’t degrade as quickly, but the subjective quality of it starts to degrade, so we use about a year as a rule of thumb.”

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Terpenes are the compounds in cannabis that give the plant its “skunky” smell. Greenblatt notes that a lack of smell of the dried flower can be a good indicator of whether the marijuana has degraded, something that happens over time if not stored properly.

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“After a certain amount of time, the terpenes will degrade, you will notice a change in the smell over time,” Greenblatt said. “After a very long time, like five years for dried flower it starts to turn brown because the chlorophyll evaporates.”

University of British Columbia psychology professor and cannabis researcher Zach Walsh argues that the government is not wrong to say dried pot expires, because the medicinal components of marijuana change over time as the flower degrades, making the product different from the day it was purchased.

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“It’s certainly appropriate for it to have an expiry date,” Walsh said. “The chemical components do degrade and change…One of the big benefits of legalization is that people are going to be able to have a pretty good idea of what they’re getting in their cannabis in terms of what the potency is, what are the constituents and if those change then it’s no longer what it says on the package, I would call that expired,” Walsh said.

“Not that it becomes dangerous, but it’s just no longer what it says,” Walsh said.

So what is it that changes in dried cannabis flower?

Canopy Growth explained that the THC, a cannabinoid that provides some of the health benefits of marijuana as well as the “buzz,” can degrade over time if the flower is not stored properly.

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“Cannabis is best stored in an air-tight container and left in the dark, because light degrades the THC, so it’s generally best to store it in the dark,” Greenblatt said.

Walsh said that even with proper storage of the dried cannabis, the chemicals in the flower, called cannabinoids, break down and change over time.

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“There’s a bunch of different cannabinoids, chemicals, inside herbal cannabis. Some have a somewhat similar structure and over time they’ll degrade and turn into one another,” Walsh said. “Something called CBN, cannabinol, THC and other cannabinoids can degrade into cannabinol which has a different psychoactive profile than THC, so it does change.”

The professor noted that the length of time before the change in the cannabis properties “has yet to be characterized, but it’s a matter of months.”

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Marijuana products are pictured at Eden Medicinal Society in Vancouver, Thursday, Jan. 30, 2018. The Canadian Press/Jonathan Hayward

“You would certainly expect to see some substantial changes in cannabis over a year,” Walsh said.

As for storage, some people like to keep their dried marijuana in the fridge or freezer to help contain the natural smell of the pot, something that should be considered a no-no.

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“I don’t recommend it … the trichomes on cannabis are very delicate. When you freeze cannabis, they can fall off when frozen,” Greenblatt explained.

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Trichomes are the crystal-like hairs on the flower and leaves that produce the cannabinoids and terpenes.

“You can unwittingly reduce the quality of your cannabis if you’re keeping it in a freezer because the part that contains the cannabinoids can break off more easily,” Greenblatt said.

According to Health Canada, dried marijuana should be kept in a cool, dry place and out of the reach of children and pets.

A bottle of cannabis oil with organic sunflower oil pictured on December 18, 2017. The Canadian Press/Sean Kilpatrick

Cannabis oils on the other hand can actually expire over time and should not be consumed past the expiry date.

“For oils, however, oils are a food product, cannabis resin diluted into like a sunflower oil or some kind of food oil,” Greenblatt said. “Oils can eventually go rancid, so we put a best before date for about a year for cannabis oils as well.”

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The government said the proposed plain packaging would include a packaging date of the product and “an expiry date, if one has been set.”