SEATTLE — Cannabidiol is one of the hottest supplements on the market today. The chemical, derived from the Cannabis sativa plant and better known as CBD, is now found in chocolates, gummy candies, sodas and more.

One place it shouldn’t be found: Amazon. The e-commerce giant’s policy expressly bars the sale of CBD in any product: “Listings for products containing cannabidiol (CBD) are prohibited.”

But a Washington Post investigation found that it’s possible, even easy, to buy the forbidden compound on the online retail site. Eleven of 13 items The Post purchased last month from Amazon contained CBD, according to an analysis that The Post paid Evio Labs, which specializes in analyzing products for the cannabis industry, to conduct. One product even had a small amount of tetrahydrocannabinol, or THC, the psychoactive chemical in cannabis plants that gets people high.

One of the products The Post purchased and tested came from Boulder, Colo.-based Weller, which sells a variety of CBD products on Amazon. The company designed new packaging for goods on Amazon different from what it uses on its own website. Its Dark Chocolate Coconut Bites on Amazon omits any mention of its most important ingredient.

“It’s the same product,” said Weller co-founder John Simmons. His company is simply trying to navigate a business with inconsistent rules as the CBD market moves into the mainstream, Simmons said.

None of the listings contained CBD in their descriptions. But consumers could find them by knowing which products contain CBD already — by checking reviews, social media or other websites that did their own testing.

The Post’s investigation illustrates the challenges Amazon faces in policing its platform, which has transformed into an enormous flea market supplied by millions of sellers listing hundreds of millions of items.

To keep CBD products off its site, Amazon says it deploys advanced algorithms designed to sniff out descriptions that could hint at the banned ingredient. (Amazon founder and chief executive Jeff Bezos owns The Washington Post.) Amazon removed some, but not all, of the product listings after being asked about them by The Post. The company said it was investigating with each of the sellers.

“Bad actors who attempt to undermine our store do not reflect the flourishing community of honest entrepreneurs that make up the vast majority of our seller community,” Amazon spokesman Patrick Graham said. “We move swiftly to hold bad actors accountable by removing selling privileges, withholding funds, and pursuing civil and criminal penalties.”

Amazon declined to say why it forbids the sale of CBD, other than to say it can decide what to sell like any retailer. CBD is still controversial and its legal situation is murky, with some federal agencies, states and localities prohibiting its sales.

CBD is usually derived from hemp, a crop that was legalized in a 2018 farm bill. But it remains illegal to put the compound in food, and the Food and Drug Administration has warned CBD makers several times, including last month, over violating the Federal Food, Drug and Cosmetic Act. Muddying the waters further, three states — South Dakota, Idaho and Mississippi — make no distinction in their laws between marijuana and hemp plants, meaning CBD is considered a controlled substance in those states. CBD, though, does not contain the chemical in marijuana that makes people high.

While CBD makers often claim the chemical can help with anxiety, improve sleep and reduce pain, the only legally edible version of CBD is a drug, Epidiolex, prescribed to treat seizure disorders. But because CBD is so widely available and seemingly on a path toward legalization in edible form, prosecutors rarely take action against companies that make supplements or individuals who use them.

Makers of those items who spoke with The Post offered a wide range of explanations for the presence of their CBD-containing products on the site, from mistakenly shipping the wrong item to ignorance regarding the rule. Two of the companies said that other merchants were selling products without their permission, and they worried that reviews for poor service could damage their brands if the e-commerce giant eventually does allow CBD product sales.

“We’re trying to control a problem [of unauthorized sales] that’s really, really hard to control,” said Sequoia Price-Lazarus, chief executive and founder of Seattle-based Lazarus Naturals.

More than 500 million products are available on Amazon at any given time, according to estimates by e-commerce research firm Marketplace Pulse. That’s because Amazon isn’t just a retailer, it’s a marketplace that has invited more than 2.5 million sellers to hawk those wares, by Marketplace Pulse’s measure. Creating a digital storefront on Amazon requires little more than a driver’s license and a bank statement.

By making it so easy to set up shop, Amazon has amassed so much selection that sellers often compete against one another, driving prices down across the site and helping lure shoppers.

But Amazon has sacrificed policing its site to amass that selection, according to former executives. CBD sellers aren’t the only beneficiaries of the hands-off approach. Amazon has become a haven for counterfeiters, where merchants hawk knockoffs of everything from luxury handbags to diapers. And it has faced criticism for selling products without any warnings despite federal agencies deeming those goods to be unsafe.

Lawmakers are paying attention, chastising Amazon, Facebook, Twitter and other tech giants for their seeming inability to control the huge platforms they’ve created. And Amazon is also facing regulatory scrutiny for its power in e-commerce and the sway it holds over the third-party merchants in its marketplace.

While Amazon didn’t disclose how many listings for CBD products it has found and removed, the company said its systems blocked more than 3 billion suspected bad listings last year alone.

The murkiness of federal CBD law, and the patchwork of state rules, make selling products with the compound perilous, said Jonathan Miller, general counsel for the U.S. Hemp Roundtable, an industry group. Law enforcement could target sellers to make a point.

“That’s a very real concern,” said Miller, who also served as Kentucky’s treasurer. “It’s a danger.”

Some financial institutions are also wary of processing transactions for controlled substances, for fear of running afoul of banking rules, he added. Risk-averse banks “don’t want to touch this,” Miller said.

A recent search for “CBD oil” on Amazon turned up more than 6,000 results, many of which were described as “hemp oil.” While it’s unclear how many of those actually contain CBD, reading through reviews and scanning the Web for articles and tweets points to those that appear to contain the chemical. One reviewer called Weller’s Dark Chocolate Coconut Bites the “best CBD food product I’ve found.”

Finding CBD supplements on Amazon isn’t difficult. The question-and-answer sections on the product listings offer clues. For Good Day Chocolate’s Sleep Hemp Supplement, for example, a question whether the product included CBD elicited a reply that product packing said it did. On Twitter, H3 Infusion boasted that its hemp extract is “our first 2% water soluble CBD” and included a link to its Amazon product page. And the photo of the label for Koi Naturals Spearmint Oil had “CBD” in it.

Good Day Chocolate acknowledged selling products on Amazon, but declined further comment. H3 couldn’t be reached for comment. And Koi Naturals didn’t respond to a request for comment.

One way supplement sellers evade Amazon’s detection is by never mentioning CBD in their listings. Often product pages use code words, such as “full-spectrum hemp extract,” to give buyers an indication that their items include CBD, said Elaine Kwon, founder of e-commerce management and software firm Kwontified and a former Amazon manager.

“It’s done with a wink-and-a-nod,” said Kwon, whose clients include CBD sellers who don’t sell on Amazon yet because they don’t want to run afoul of the company’s rules.

Kwon believes Amazon executives are aware of CBD sales on the site but turn a blind eye, taking “half measures” to combat the issue. “They do know it’s happening,” Kwon said. “It’s a good revenue driver. They are making a lot of money off it.”

Amazon disputes the claim.

“Amazon does not knowingly permit the sale of products it prohibits,” Graham said. “The lack of credibility of this … statement suggests this individual has no knowledge of how Amazon’s systems work.”

The Post spent $566 buying nine tinctures and four edibles from Amazon that had a high probability of containing the chemical because of mentions of CBD in the products on social media, in reviews and elsewhere. Eight of the products were shipped directly from Amazon’s warehouses, rather than from sellers’ own facilities. The Wellers bites even carried the “Amazon’s Choice” badge, a label the company uses to recommend products. Amazon’s Graham said the company considers a variety of factors, including popularity, product availability and customer reviews, to award the designation.

Evio, the testing company, found that one of the tinctures, a mint-chocolate-flavored oil from Longmont, Colo.-based Restorative Botanicals had 12.5 milligrams per milliliter of CBD. But it also included 0.3 milligrams per milliliter of THC, a substance that Amazon also prohibits from its site.

Restorative is following federal guidelines, which don’t require manufacturers to mention CBD on its labeling, said President Bernard Perry. Instead, his company’s products describe themselves as “full-spectrum hemp-oil” supplements. Perry declined to comment on whether Restorative followed Amazon’s rules with regard to CBD sales.

“We stay in compliance as much as we can,” Perry said.

And just as some products hide their CBD from Amazon, other items suggest they include the compound but actually don’t have any of the chemical in them. One of the two products The Post tested that didn’t include CBD was a tincture from Tranquilo Essentials, a company whose Twitter handle was @AmazonBestCBD. The company shut down the account after The Post asked about it.

A person responding to an email to the company’s website said that the answer was that the product’s name contains “hemp oil.” The person didn’t respond to subsequent questions.

Sentia Wellness, a Portland, Ore., maker of CBD products, doesn’t sell directly on Amazon to abide by the e-commerce company’s rules, said President Angelo Lombardi. But The Post purchased its Select Oil, which had 29.9 milligrams per milliliter of CBD in it, which was sold by a third-party merchant — a violation of Sentia’s rule that distributors aren’t allowed to sell to authorized retailers.

But Amazon’s inability to control CBD sales is so vexing that Sentia might eventually choose to break the rules, too. Amazon’s lax oversight is providing Sentia’s rivals an opportunity that it’s missing out on because it has chosen to follow Amazon’s rules, Lombardi said.

“The statement is they don’t sell CBD. The reality is that they sell CBD,” Lombardi said. “We’re missing a chance to get our brand out there.”