The country’s top drug regulators on Wednesday pushed back on claims they move too slowly in approving new stem cell treatments and sought to temper expectations for the field’s potential to benefit patients, emphasizing the hazards and the unknowns that persist with developing such therapies.

“To ensure that this emerging field fulfills its promise to patients, we must first understand its risks and benefits and develop therapeutic approaches based on sound science,” Food and Drug Administration officials wrote in the New England Journal of Medicine. The paper was written by FDA Commissioner Dr. Robert Califf along with Drs. Peter Marks and Celia Witten, the top two officials in the FDA’s Center for Biologics Evaluation and Research.

The authors make their argument in the staid language befitting both government regulators and academic journals. But their message is clear: Let’s all take a deep breath on the stem cell hype.

Excitement for stem cell treatments has been growing as scientists have learned more about how the cells could be coaxed into replacing damaged or dead cells throughout the body, with possible treatment applications for a wide range of injuries and diseases. But for now, the FDA officials said, the excitement is just that, echoing what many stem cell scientists have said about the field.

Read more: After criticism from scientists, Congress eases its pursuit of faster stem cell therapies

So far, studies “have not reliably demonstrated the effectiveness of stem-cell treatments even in some of the most systematically studied conditions, such as heart failure and graft-versus-host disease,” the authors wrote. Plus, it’s not as though the cells are so ingenious that they can be placed anywhere in the body — “whether injured knee cartilage or a neurologic deficit” — and function as needed.

The FDA officials also offered a strong argument for preserving the gold-standard clinical trials that new treatments typically need to pass before being approved. Only those trials, and not anecdotes or patient belief, can show that treatments are safe and effective, they said.

Even scientists who have urged caution on stem cell treatments have said the FDA could make it easier for therapeutic candidates to enter clinical trials. But the agency officials stalwartly defended their approach.

The notion that their standards for approving stem cell therapies are too rigorous stems from the fact that researchers developing new therapies don’t understand all the pathways available to them, Califf and his colleagues wrote. They said they are committed to getting safe and effective treatments to patients and are willing to work with researchers under the FDA’s programs that can expedite the regulatory process.

Interestingly, the programs the authors mentioned include the accelerated approval process that can get drugs to the market before a clinical benefit is demonstrated. Tucked into the 21st Century Cures Act, a 1,000-page bill Congress is considering this week, is a provision that would allow the FDA to grant regenerative medicine products, including stem cell therapies, accelerated approval.

In the New England Journal paper, the FDA officials also alluded to the largely unregulated use of stem cells by hundreds of clinics. Those treatments are often offered without evidence that they are safe or work, “sometimes with the claim that they constitute revolutionary treatments for various conditions,” the authors said.

In September, the FDA held a meeting about its proposal to regulate those cells as drugs, and in the paper, the officials reiterated their concerns about what some experts have dubbed a Wild West of unmonitored stem cell treatments.

Many of the clinics use stem cells from the patients themselves — what are called autologous stem cells. The FDA officials said that while using those cells is typically safer than using donor stem cells, some patients have grown tumors or gone blind after treatments.

Such “adverse effects” are probably more common than known because officials can’t track them if they are happening outside of regulated trials — yet another reason they argued for well-conducted studies of potential treatments.

“Without such studies,” the officials wrote, “we will not be able to ascertain whether the clinical benefits of such therapies outweigh any potential harms.”