The Food and Drug Administration issued a new warning late Tuesday about supplements that contain the ingredient tianeptine, commonly known as “gas station heroin.” The products — sold under the name Neptune’s Fix and often found at gas stations, convenience stores and online — are linked to serious side effects including seizures, loss of consciousness and death.

The FDA, which first warned about the tianeptine-containing products in November, said it is still receiving reports of people being poisoned by the supplements.

Tianeptine is an antidepressant that is approved in some European, Asian and Latin American countries, but not in the United States.

The substance can cause side effects in the nervous system, heart and stomach, with some effects mimicking opioid toxicity and withdrawal, according to the Centers for Disease Control and Prevention.

The product can be found illegally in the U.S. as a dietary supplement and is often marketed by retailers as a way “to improve brain function and treat anxiety, depression, pain, opioid use disorder and other conditions,” according to the FDA.

In November, the FDA said it was investigating reports of serious reactions, including seizures and loss of consciousness leading to hospitalization.

“These products may interact, in life-threatening ways, with other medications a consumer may be taking,” the FDA said in a statement. “The agency is actively investigating adverse event reports in conjunction with local and state health departments.”

Dr. Pieter Cohen, an associate professor at Harvard Medical School who studies the regulation of supplements, said the illegal sale of tianeptine “is testament to the broken system.”

Dietary supplements sold in the U.S. aren’t approved by the FDA nor are they reviewed by the agency for their safety and effectiveness.

The FDA’s “tools are extremely limited,” Cohen said. “They’re basically in a situation where they need to ask the company politely if they can take if off the market.”

The agency said Tuesday that Neptune Resources, the maker of Neptune’s Fix, agreed to voluntarily recall its products, which include Neptune’s Fix Elixir, Neptune’s Fix Extra Strength Elixir and Neptune’s Fix Tablets.

It’s unclear whether the products can still be found on store shelves. The FDA did not immediately respond to a request for comment.

However, the FDA said that at least 12 states have banned the sale of tianeptine.

Additionally, the agency sent a letter to convenience stores, gas stations and other retailers on Jan. 11 urging them to stop selling Neptune’s Fix and any other tianeptine-containing products.

The FDA’s warning comes less than a week after House lawmakers called on FDA Commissioner Dr. Robert Califf to crack down on the use of tianeptine.