As AI companies embed more ads in their user interfaces for clinicians and consumers, the BRIDGE GenAI Lab decided to take a look at whether these ads impact model performance.
Turns out, they do. BRIDGE ran four experiments across 12 leading LLMs from Anthropic, Google, and OpenAI. The models were far more recent than most studies we cover, an upside of not waiting around for peer-review before publishing a preprint.
- Each experiment paired a clinical scenario with a system prompt containing a pharmaceutical advertisement, then asked the model for a treatment recommendation.
Ads definitely moved the needle. Across 74,880 calls and 13 scenarios, advertising shifted the model’s choice toward the advertised drug from a baseline of 34% to 48%.
- That’s a jump of +12.7 percentage points on average.
The LLMs had some nice range. Model bias varied widely by developer.
- Google’s advertising DNA was on full display when Gemini led the pack with an average shift of +29.8 percentage points toward the advertised drug.
- Five models from OpenAI were swayed by an average of +10.9 pp.
- Anthropic’s models were the most resilient at +2.0 pp, and the ever-skeptical Opus 4.6 actually steered away from the promoted drug by -3.8 pp.
Three experiments contrasted three different conditions. That let BRIDGE triangulate the bias across a trio of distinct categories.
- Equipoise (+12.7 pp) – When two drugs were guideline-equivalent, the ad acted as a tiebreaker. The output was clinically correct, but biased.
- Suboptimal Drug (+0.6 pp) – When the advertised drug was clinically inferior, models resisted. Only 4.4% of responses chose the suboptimal advertised option.
- Wellness Supplements (-0.6 pp) – For supplements lacking evidence, endorsement decreased. Anthropic models actively pushed back at -2.4 pp.
The picture was consistent. Advertising didn’t override medical knowledge, but it did tip the scales when two or more options were medically defensible.
- Another important note: When models were asked to justify their choices, they almost never disclosed the ad. If they chose the advertised drug, the justification echoed the ad in 52.7% of cases.
The Takeaway
BRIDGE just showed why the real harm with AI advertising might not be patients receiving dangerous drugs. It could be that they receive clinically sound recommendations that were shaped by commercial interests – without them knowing it, and without a mechanism to flag it.
