The company says physicians sent more than 42 million prompts last month, using the chatbot to search medical journals and clinical guidelines to help diagnose and treat patients.
In September it raised $250 million at a $15 billion valuation, and it has drawn nearly $1 billion in investment over the past 18 months from backers including Andreessen Horowitz, Blackstone, Thrive Capital and Sequoia.
Nadler told the Financial Times it is "the first viral consumer internet app in healthcare," and the company has grown largely by word of mouth.
Hospitals including Mount Sinai, Memorial Sloan Kettering and NYU Langone have built ties with it, embedding it in electronic health records.
NYU Langone's Paul Testa said the system sought a formal arrangement after finding clinicians already using it on their own.
Yet research on its accuracy is mixed.
A Mayo Clinic study, from an investor in the company, found accurate, evidence-based recommendations in all five sampled cases, though its effect on treatment decisions was limited.
A June paper in Nature found it underperformed general-purpose models such as Google's Gemini and OpenAI's GPT on clinical benchmarks.
There are also concerns that junior doctors may lean on such tools before mastering the basics.
Next, Nadler said the company will start clinical trials in the coming weeks with researchers, targeting cancer and rare diseases, and plans to apply AI to drug discovery by mining existing scientific literature. It is also expanding into ambient documentation, which means note-taking during patient visits. Competition is growing: OpenAI offers ChatGPT for healthcare, and Anthropic has released healthcare tools, though it recently partnered with OpenEvidence to bring them to 100 low- and middle-income countries. Critics question whether the advertising-based model justifies the valuation, and the company's withdrawal from the EU and UK earlier this year over the AI Act has limited growth in those markets.