Google’s search feature AI Overviews cites YouTube more than any medical website when answering queries about health conditions, according to research that raises fresh questions about a tool seen by 2 billion people each month.
The company has said its AI summaries, which appear at the top of search results and use generative AI to answer questions from users, are “reliable” and cite reputable medical sources such as the Centers for Disease Control and Prevention and the Mayo Clinic.
However, a study that analysed responses to more than 50,000 health queries, captured using Google searches from Berlin, found the top cited source was YouTube. The video-sharing platform is the world’s second most visited website, after Google itself, and is owned by Google.
Researchers at SE Ranking, a search engine optimisation platform, found YouTube made up 4.43% of all AI Overview citations. No hospital network, government health portal, medical association or academic institution came close to that number, they said.
“This matters because YouTube is not a medical publisher,” the researchers wrote. “It is a general-purpose video platform. Anyone can upload content there (eg board-certified physicians, hospital channels, but also wellness influencers, life coaches, and creators with no medical training at all).”
Google told the Guardian that AI Overviews was designed to surface high-quality content from reputable sources, regardless of format, and a variety of credible health authorities and licensed medical professionals created content on YouTube. The study’s findings could not be extrapolated to other regions as it was conducted using German-language queries in Germany, it said.
The research comes after a Guardian investigation found people were being put at risk of harm by false and misleading health information in Google AI Overviews responses.
In one case that experts said was “dangerous” and “alarming”, Google provided bogus information about crucial liver function tests that could have left people with serious liver disease wrongly thinking they were healthy. The company later removed AI Overviews for some but not all medical searches.
The SE Ranking study analysed 50,807 healthcare-related prompts and keywords to see which sources AI Overviews relied on when generating answers.
They chose Germany because its healthcare system is strictly regulated by a mix of German and EU directives, standards and safety regulations. “If AI systems rely heavily on non-medical or non-authoritative sources even in such an environment, it suggests the issue may extend beyond any single country,” they wrote.
AI Overviews surfaced on more than 82% of health searches, the researchers said. When they looked at which sources AI Overviews relied on most often for health-related answers, one result stood out immediately, they said. The single most cited domain was YouTube with 20,621 citations out of a total of 465,823.
The next most cited source was NDR.de, with 14,158 citations (3.04%). The German public broadcaster produces health-related content alongside news, documentaries and entertainment. In third place was a medical reference site, Msdmanuals.com with 9,711 citations (2.08%).
The fourth most cited source was Germany’s largest consumer health portal, Netdoktor.de, with 7,519 citations (1.61%). The fifth most cited source was a career platform for doctors, Praktischarzt.de, with 7,145 citations (1.53%).
The researchers acknowledged limitations to their study. It was conducted as a one-time snapshot in December 2025, using German-language queries that reflected how users in Germany typically search for health information.
Results could vary over time, by region, and by the phrasing of questions. However, even with those caveats, the findings still prompted alarm.
Hannah van Kolfschooten, a researcher specialising in AI, health and law at the University of Basel who was not involved with the research, said: “This study provides empirical evidence that the risks posed by AI Overviews for health are structural, not anecdotal. It becomes difficult for Google to argue that misleading or harmful health outputs are rare cases.
“Instead, the findings show that these risks are embedded in the way AI Overviews are designed. In particular, the heavy reliance on YouTube rather than on public health authorities or medical institutions suggests that visibility and popularity, rather than medical reliability, is the central driver for health knowledge.”
A Google spokesperson said: “The implication that AI Overviews provide unreliable information is refuted by the report’s own data, which shows that the most cited domains in AI Overviews are reputable websites. And from what we’ve seen in the published findings, AI Overviews cite expert YouTube content from hospitals and clinics.”
Google said the study showed that of the 25 most cited YouTube videos, 96% were from medical channels. However, the researchers cautioned that these videos represented fewer than 1% of all the YouTube links cited by AI Overviews on health.
“Most of them (24 out of 25) come from medical-related channels like hospitals, clinics and health organisations,” the researchers wrote. “On top of that, 21 of the 25 videos clearly note that the content was created by a licensed or trusted source.
“So at first glance it looks pretty reassuring. But it’s important to remember that these 25 videos are just a tiny slice (less than 1% of all YouTube links AI Overviews actually cite). With the rest of the videos, the situation could be very different.”
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