Amazon's product recommendation engine may not always be trustworthy.

The company applies the "Amazon's Choice" badge to some products that are unsafe, mislabeled and violate its own policies, a Wall Street Journal investigation has found. The label appeared on dozens of products that were banned, didn't meet safety standards and featured fake safety certifications. It was also applied to controlled substances, like steroids and marijuana products, the Journal reported.

In other cases, "Amazon's Choice" listings were manipulated with specific keywords that would ensure they'd be included in the recommendation engine. The Journal discovered some third-party merchants have developed ways to game the algorithms that help determine which products are featured, by pushing consumers to buy an item, which artificially juiced sales and made it appear more popular.

An Amazon spokesperson said the company doesn't tolerate policy violations, including review abuse, incentivized reviews, counterfeits or unsafe products.

"When deciding to badge a product as Amazon's Choice, we proactively incorporate a number of factors that are designed to protect customers from those policy violations," the spokesperson said. "When we identify a product that may not meet our high bar for products we highlight for customers, we remove the badge."

Amazon introduced the "Amazon's Choice" service in 2015 as a way to suggest products to shoppers who made purchases using Alexa, its voice-controlled assistant. The company said the badge would appear next to select "highly rated, well-priced" items. However, few details have been shared about how Amazon chooses those products.

The company has faced growing concerns that the "Amazon's Choice" badge could be used to promote low-quality items. In August, Sen. Bob. Menendez (D-NJ) and Sen. Richard Blumenthal wrote a letter to Amazon CEO Jeff Bezos asking for more details on how the program works, after a BuzzFeed report found it often endorses defective or inferior items.

The Wall Street Journal report is the latest to show how Amazon struggles to keep tabs on its massive marketplace. Earlier this year, CNBC found that expired food products are sold through Amazon by third-party sellers. In a separate investigation, The Wall Street Journal discovered thousands of unsafe products sold on Amazon.

Read the full investigation in The Wall Street Journal.