Instagram has blocked searches for certain terms associated with the suspected illegal sale of drugs via its service.
The photo-themed social network took the measure after being asked to respond to an investigation by #BBCtrending - a new social media series.
The journalists had uncovered many pictures and videos of narcotics posted alongside text advertising their sale.
Instagram is owned by Facebook.
The firm has a policy of acting on posts reported as being inappropriate, but it believes it would be impractical and invasive to search for such material.
"Instagram has a clear set of rules about what is and isn't allowed on the site," a spokeswoman told the BBC.
"We encourage people who come across illegal or inappropriate content to report it to us using the built-in reporting tools next to every photo, video or comment, so we can take action.
"People can't buy things on Instagram, we are simply a place where people share photos and videos."
Among Instagram's "report photo/video" choices is the option to identify suspected drug use.
The BBC understands Facebook's staff aim to review posts flagged to either of its social networks within 48 hours. They also have the option of blocking terms classed as "bad hashtags" - ones that promote banned activities - if they are mentioned in the press or in user reports.
The only content Facebook does actively search for is images of child abuse.
Most of the drugs-related activity appears to be taking place in the US.
"Just getting a few packs ready for tomorrow morning... Place your order today, it gets shipped out at 8AM tomorrow," read one post placed beneath an image of bags of marijuana.
Another picture showed a variety of pills, adding: "$2 a pop for xans, $10 a pop for roxys."
This refers to Xanax, a psychoactive anxiety treatment, and Roxicodone, an opiate used to treat pain.
Both require prescriptions in the US and the UK, but are sometimes bought on the black market.
Crystals of MDMA and other amphetamine-related substances were among other drugs advertised via photos and videos.
In many cases the buyer and seller arranged to finalise their deals using WhatsApp or Kik - instant messaging apps in which they could keep messages private. Like Instagram, accounts can be set up on these services without revealing either party's true identity.
Instagram is not the only social network on which drugs are advertised.
The BBC has also seen instances of the practice in comments below some videos on Google's YouTube service.
But while it is relatively common for the person who uploaded a drug-themed photo or video on Instagram to be the one advertising the sale of the substance, on YouTube the person posting the ad tends to do so below videos belonging to others.
Like Facebook, Google relies on users reporting a problem before taking action.
"We take user safety seriously and have guidelines that prohibit any content encouraging dangerous, illegal activities," said a spokeswoman for YouTube.
"This includes content promoting the sale of drugs. YouTube's review teams respond to videos flagged for our attention around the clock, removing millions of videos each year that violate our policies."
One drugs abuse researcher - who has advised the UN, World Health Organization and the UK government - said he was concerned by what he had seen.
"I'm not particularly sophisticated on the internet, and it took me 10 seconds to see posts selling class-A drugs on Instagram," said Prof Neil McKeganey, founder of the Centre for Drug Misuse Research, in Glasgow.
"Here is a public space being used to trade some of the most dangerous substances that we know are being abused.
"I absolutely feel there is a responsibility to take proactive action.
"It seems to me far too serious for those who own the companies that provide the public space through which this is occurring to simply say it's up to contributors to bring this to their attention."
UK-based drugs treatment charity Addaction said it too wanted social media companies to act "swiftly and vigilantly", adding that it believed the companies could make a positive difference if they did.
"Social media is a great way of reaching out to millions and millions of people," said spokesman Elliot Elam.
"That's why we'd like to see providers of these sites work with organisations like ours, so they can find ways to engage with any users who may be struggling with drug or alcohol problems."
Google and Facebook are not the only companies that rely on user reports to indentify potential drugs deals.
Yahoo's blogging service Tumblr confirmed it had the same policy.
"For legal reasons, we do not proactively monitor the site," said a spokeswoman.
"We respond to reports of activity that is illegal or against our policies pursuant to those policies and relevant law."
Although the BBC found photos of illegal drugs on Tumblr, searches for the terms that brought up associated adverts on Instagram did not appear to do so on Yahoo's service.
This is not the first time Instagram's self-policing policies have been called into question.
In August the Fusion.net blog suggested that the illegal psychedelics 2C-I and 2CB were also being advertised via the app.
Vice Magazine also highlighted that marijuana and several kinds of prescription pills were promoted on the service.
However it suggested that banning related hashtags would not solve the problem, saying "users would get more creative and choose other labels".
More recently US senator Edward Markey wrote to the service's chief executive, Kevin Systrom, asking him to look into reports that unregulated gun sales were being conducted through the app.
"Other companies that enable online sales have enacted commonsense protocols," he wrote.
"I encourage Instagram to take similar steps and adopt safe business practices that curb the marketing and sale of guns."
#BBCtrending is a hand-picked selection of stories trending on social media around the world. Have you seen an interesting trend? Tweet us.
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