These plants at the Oakland Museum of California are identified as Cannabis indica... or are they? (Ben Margot / TheAssociated Press files)

How do people know what kind of cannabis they want to use?

Some of Canada's new legal cannabis stores are answering that question by falling back on a popular tradition: dividing different types of cannabis into sativa, indica, or a mix of both.

According to this convention, these two kinds of cannabis have different effects on human physiology. (Indica is supposed to be a more sedative "body high" and sativa is a more energizing "head high." So-called "hybrid" cannabis, of course, lies somewhere in between.)

The indica-sativa split is a simple way for cannabis users to understand the plant, which helps explain its popularity. But as a recent review of cannabis classifications reminds us, the truth about cannabis classification is more complex than that comforting dichotomy— and much, much more confusing.

Most of John McPartland's October paper in the journal Cannabis and Cannabinoid Research sums up esoteric arguments about cannabis taxonomy that date back hundreds of years.

Here's the very short version: Carl Linnaeus, the Swedish scientist who founded the modern system of classifying plants and animals, identified Cannabis sativa as a species in his 1753 smash hit publication Species Plantarum.

In 1785, the French naturalist Lamarck proposed Cannabis indica as a separate species based on differences he observed in the plant's physical and chemical characteristics.

The acolytes of these two 18th-century figures carried on this taxonomic tiff for years to come, explains McPartland. The key argument involves whether the two kinds of cannabis plants are truly different species, or both subspecies of Cannabis sativa. It's a distinction that's notoriously subjective, McPartland writes.

But based on research using DNA barcoding, McPartland argues that Cannabis sativa and Cannabis indica are so genetically similar that they should definitely be considered subspecies, or varieties, of the single species Cannabis sativa.

McPartland says decades of field botanists haphazardly assigned the terms "indica" and "sativa" to cannabis specimens collected in different parts of the world, confusing the issue and "ultimately giving rise to today's vernacular taxonomy of sativa and indica, which totally misaligns with formal C. sativa and C. indica."

Along the way, cannabis users picked up on those terms and created what McPartland describes as an inaccurate "folk taxonomy" of cannabis.

At this point, McPartland argues, the difference is actually inconsequential: "Categorizing cannabis as either sativa and indica has become an exercise in futility," he writes. "Ubiquitous interbreeding and hybridization renders their distinction meaningless."

If you want to learn more, it's worth taking the time to read McPartland's paper.

If not, know this: When you walk into a legal cannabis store and start browsing cannabis strains based on their purported status as sativa or indica, you're entering murky territory.

As this Leaf News feature explains, researchers have even determined that some purported "pure sativa" and "pure indica" strains are nearly identical from a genetic perspective. There are clear botanical differences between these two types of cannabis plant, but it's not clear whether those differences actually correspond to different effects on human physiology.

In time, it seems likely that researchers will develop new ways to describe different types of cannabis based on genetic and chemical analysis, but until then, the mythology of indica and sativa doesn't look like it's going anywhere.

If you find that upsetting, we recommend you take a deep breath and relax — perhaps a nice, calming indica strain would help?