Berlin - TikTok is using word filters to suppress comments with certain words like "gay" and "LGBTQ" without users noticing, according to research carried out by German media.

Comments featuring common drugs such as cannabis, crack, heroin, cocaine and LSD are also being suppressed on the video app, regardless of context, according to research by broadcasters NDR and WDR and news service Tagesschau.

The Chinese-owned video app, which has grown rapidly to rival the likes of Instagram and YouTube thanks to a highly addictive user experience, does not make it transparent to users if and why comments do not appear, according to the journalists, whose tests were carried out in Germany.

Instead, the app is giving users the impression that their comments are public, a technique known as "shadowbanning". TikTok users can therefore not be entirely sure whether their own comment is visible to other people.

Among at least 20 words being blocked by TikTok in Germany are also "Nazi", "slaves" and "gas" in what the researchers called a restriction on the freedom of speech.

A TikTok spokesperson admitted to the researchers that the company uses technology to "proactively" search for comments that violate TikTok guidelines or that constitute spam behavior. They also try to prevent users from posting the same comment more than once.

In the current instances, comments that did not violate the community guidelines had also been wrongly flagged as potentially harmful. "This should not have happened and we will continue to train our automated systems to ensure fair and consistent moderation," the TikTok spokesperson said.

The researchers had already reported last March that posts by users containing certain words were being suppressed on TikTok. Among the terms blocked at the time were the words "Auschwitz" and "National Socialism," which are now apparently no longer blocked.