Facebook’s ad sales team is hitting up the drug industry.

On Thursday, the company’s New York-based health unit hosted an invite-only breakfast for pharmaceutical marketers to learn about targeting users for their clinical trials.

CNBC viewed a copy of the invitation, which asked participants to attend a presentation on the company’s “new clinical trials strategy.”

Facebook is already widely used by clinical trial recruiters, and it’s a massive revenue opportunity for the company. Research firm eMarketer estimates that pharma and health care marketers will spend $3.1 billion on digital advertising by 2020, up from $1.9 billion last year.

According to a person who attended Thursday’s event, Facebook detailed how drug marketers can and can’t target users. The source requested anonymity as Facebook did not make the details public.

Facebook’s health team explained that users can’t be targeted based on health conditions like insomnia. This is not limited to clinical trials.

Marketers can target people by demographics and their expressed interests, or likes. Millions of health groups have organically popped up on Facebook for people with a wide variety of health conditions, though marketers can’t use that data in their outreach.

Some drug companies have been reluctant to use Facebook to date due to concerns that patients will share sensitive information like medical side effects and adverse events. Pharmaceutical companies are required to monitor and report these comments.

This wasn’t Facebook’s first event for drug marketers. CNBC reported in May that the company was hosting a summit the following month to pitch its platform as an alternative to traditional television and print media ads.