A long-running dispute over the First Amendment flared up this past Friday, when a federal judge in New York told FDA that it couldn’t prevent a drug manufacturer from discussing research supporting an off-label use of its drug.

The drug at issue is Vascepa, the active ingredient of which is an omega-3 fatty acid. It’s similar to fish oil, so let’s just call it “fishy oil.” In 2012, FDA approved the fishy oil for use in people with very high triglyceride levels (above 500 mg/dL). Turns out that taking fishy oil reduced their triglycerides, which are associated with health disease.

The manufacturer of the fishy oil, Amarin, thought that people who had persistently high, but not very high, triglycerides (between 200 and 499 mg/dL for those already on statins) might benefit, too. So, with FDA’s blessing, Amarin came up with a study to see if they did. And it turns out their triglycerides also dropped.

The thing is, though, that the ultimate goal of the fishy oil isn’t to reduce triglycerides. It’s to prevent heart disease. And what if reducing triglycerides doesn’t help with that, as some recent studies have suggested? Sure, triglycerides are associated with heart disease. But it could be that high triglycerides aren’t the problem. Maybe something else associated with high triglycerides is the problem.

FDA thus refused to approve the fishy oil for use in those with persistently high (but not very high) triglycerides. It wanted more evidence that reducing triglycerides in this population actually prevented heart disease. Studies to test that are ongoing; we should know more in a few years.

In the meantime, however, doctors are prescribing this fishy oil to lots of people who don’t have very high triglycerides. And that’s OK: under the law, prescribing drugs for off-label uses is 100% legal and very common. According to Amarin, about half of all prescriptions for fishy oil are for off-label uses.

That’s where the First Amendment comes in. Amarin wants to talk to doctors about the studies showing that its fishy oil reduces triglycerides. It doesn’t want to claim that fishy oil prevents heart attacks, which it hasn’t yet shown to FDA’s satisfaction. It just wants to convey truthful, non-misleading information about the effect that its product has on triglycerides—without adhering to a bunch of persnickety FDA rules about appropriate ways to communicate that information. And it says it’s got a First Amendment right to do so.

FDA doesn’t see it that way. It’s not naïve. It knows that Amarin isn’t sharing information about fishy oil out of the goodness of its heart. Amarin wants doctors to prescribe it for off-label uses. And the agency thinks that promoting off-label uses undermines the integrity of its review process. If manufacturers were free to tout off-label uses, they wouldn’t have much incentive to seek FDA approval of those uses. Since rigorous testing sometimes shows that off-label uses are ineffective or harmful, rampant evasion of FDA approval could end up hurting patients.

In FDA’s view, the public interest in a robust drug-approval process outweighs any First Amendment concerns. And the agency isn’t all that worried about stifling debate. Doctors and researchers can talk all they want about fishy oil. FDA is just telling Amarin—no one else—to zip it.

The fishy oil case thus presents a familiar clash between a strong governmental interest and a strict limitation on commercial speech. That kind of clash can raise genuinely hard questions of constitutional law. But the court here didn’t have to break much ground in ruling against FDA. A few years back, the Second Circuit, in United States v. Caronia, found that a drug manufacturer had a First Amendment right to convey truthful, non-misleading information about its product. Because the Second Circuit’s decisions bind federal judges in New York, Caronia all but dictated the outcome in the fishy oil case.

What happens next? FDA may appeal to the Second Circuit, but Caronia suggests it’ll probably lose. And I’m skeptical the government will ask the Supreme Court to intervene. The current Court has an absolutist streak when it comes to the First Amendment, and it won’t take kindly to a government restriction on truthful speech. That’s probably why the government didn’t appeal Caronia when it was decided.

From the drug companies’ perspective, however, the fishy oil case shows why Caronia is almost as good as a Supreme Court decision. Anytime FDA threatens to penalize a drug company for sharing information about off-label uses, the company can run to a federal court in the Second Circuit to get an injunction. Even if the company is headquartered elsewhere, all it has to do is find a doctor in New York to serve as a co-plaintiff. Caronia may technically apply only in the Second Circuit, but the decision has national effects.

That’s why Caronia and the fishy oil case are especially big problems for FDA. Drug manufacturers still can’t promote off-label uses with impunity. But they’re one big step closer.