Tianeptine, found at convenience stores, at smoke shops and online, can mimic an opioid. It is among a growing class of substances that are difficult to control.

The young father headed across the parking lot to join the other parents meeting their children’s new preschool teachers. After a few steps, he began sweating and twitching. As the sky reeled, he staggered back to the car, desperate to lie down in the back seat and breathe, hidden by tinted windows.

“Did you take something?” his wife, Anne, shouted at him while dialing 911. Eric, 26, had completed rehab earlier in the summer.

“The shot! The shot!” he groaned, just before he hit the ground and blacked out.

In the emergency room of a nearby hospital in southern New Jersey, doctors tried to revive him with a defibrillator.

“What’s he on?” they yelled at Anne.

She showed them a shot-size bottle of the cherry-flavored elixir she had fished out of the car. It was labeled Neptune’s Fix. Eric had bought it at a local smoke shop.

“What the hell is that?” a doctor asked.

Neptune’s Fix features an ingredient called tianeptine — popularly known as gas-station heroin.

Often sold as a dietary supplement and promoted by retailers as a mood booster and focus aid, tianeptine is among a growing, unregulated class of potentially addictive products available in gas stations, convenience stores and smoke shops and across the internet. They typically include synthetic pharmaceuticals and plant-derived substances.