PubMed jumped from 64th to fourth place among the domains ChatGPT cites most often in U.S. answers between March and September, while WebMD fell from 18th to 79th, according to Ahrefs data. The shift marks a clear tilt toward primary sources over sites that summarize them — and it's measurable.

Ahrefs analyzed the 200 most-cited domains in ChatGPT's U.S. answers, drawing on 2.4 million answers in March and 3.8 million in September. The pattern was consistent: government health and research sites, regulators, and hands-on product testers climbed, while health and finance explainers lost ground. Britannica, Investopedia, and the New York Post dropped out of the top 200 entirely.

Ahrefs put it plainly in the report: ChatGPT "seems to be prioritizing original sources over sites that summarize them."

Which sites climbed and which fell?

Ahrefs characterizes WebMD and Verywell Health as health sites that summarize research, and groups Healthline and Investopedia alongside them as health and finance explainers. All of them slipped. On the way up: regulators such as the CFPB (Consumer Financial Protection Bureau) and FTC, plus support pages from Google and Microsoft.

There's a methodological catch, and Ahrefs addressed it head-on. The tracking system adds prompts over time, so a rank change might reflect new questions entering the set rather than a genuine change in what ChatGPT cites. To rule that out, Ahrefs compared roughly 25,000 prompts that appeared among its 150,000 highest-volume prompts in both March and September. Every site on the chart moved in the same direction for those overlapping prompts. The trend held.

Why did Reddit and Wikipedia citations drop?

Reddit's decline is the sharpest number in the dataset. ChatGPT cited Reddit in 19.1% of U.S. answers in March; by September that fell to 2.3%. Wikipedia slid from 14.1% to 2.9% over the same period. Ahrefs notes both remain among ChatGPT's most-cited domains despite the drop.

This isn't the first signal of a Reddit decline. Promptwatch data from August showed reddit.com's share of ChatGPT Search citations falling from an average of 3.83% in late July and early August to 0.52% by mid-August. Ahrefs measures something different — the share of answers citing Reddit — but the direction matches.

One nuance: in Ahrefs' separate worldwide sample, reddit.com was still the ninth domain ChatGPT searched most often using site: searches, which restrict a search to a single website.

How often does ChatGPT actually search the web?

Most free answers don't. Out of 66.6 million free, logged-out responses from August and September, only 14.4% involved a web search. That sample covers all countries. Responses that did search averaged around 1.6 fan-out queries — the background searches ChatGPT runs before answering.

Of the 15 million fan-outs logged, 6.8% were site: searches spread across 116,000 domains. Among site: searches hitting the roughly 160 most-searched domains, 75.9% targeted brand sites — company websites like apple.com or ikea.com, in Ahrefs' definition.

Other studies report much higher fan-out rates, and the report flags the discrepancy. Nectiv found an average of 7.61 fan-outs per prompt across about 4,000 prompts processed through GPT-5.6 Sol via the API. The two figures aren't directly comparable: different models, different prompts, different collection methods. Ahrefs' 1.6 average also only counts responses that involved searching at all, and the company notes its prompts span search demand across every topic and country. Free, logged-out use also triggers fewer searches than paid or reasoning models.

OpenAI's own help page says ChatGPT may search the web on its own when a question would benefit from current information — that's the confirmed engine-side statement here; the citation rankings are third-party tool data.

What should publishers take from this?

For health and finance explainer sites — Healthline, WebMD, Investopedia — the data shows eroding citation rank. For PubMed, the FDA, and testing sites like RTINGS, it shows gains. Ahrefs' recommendation: focus on pages only you can create — original research, hands-on testing, your own product information.

One caveat worth repeating: citation rank isn't traffic. If explainer pages carry your visibility, the practical move is checking which sources ChatGPT cites for your core topics, and doing that again whenever OpenAI ships a model update. Ahrefs says its fan-out data reflects a single-model snapshot from August to September, and these patterns could change in future updates.