The photo sharing service Instagram has taken steps to block certain hashtags in a bid to stamp out the sale of illegal drugs via its users.

The social networking site – where people post and comment on photographs and videos – announced the move after a BBC investigation, which found users were showing narcotics and advertising their sale.

Most of the drug sales appear to have taken place in the US, according to the #BBCtrending strand on the broadcaster's website.

Among the illegal drugs advertised on Instagram were MDMA and other amphetamine-related substances.

One ad posted below an image of bags of marijuana said: "Just getting a few packs ready for tomorrow morning … Place your order today, it gets shipped out at 8am tomorrow."

Another photo displayed a selection of pills with the caption: "$2 a pop for xans, $10 a pop for roxys " – a reference to Xanax, a psychoactive anxiety treatment, and Roxicodone, an opiate used to treat pain.

Both drugs are available on prescription in the US and the UK, but are also traded on the black market.

The BBC reported that many deals were finalised via instant messaging apps such as WhatsApp or Kik, allowing messages to be kept private.

Instagram, which is owned by Facebook, has a policy of acting on reported inappropriate activity.

But it told the BBC it would be impractical and invasive to actively search for such material.

Its terms of service state: "You may not use the service for any illegal or unauthorised purpose."

A spokeswoman told the BBC: "Instagram has a clear set of rules about what is and isn't allowed on the site.

"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."