A packet of K2, a synthetic marijuana-like product that was sold in Lawrence in 2009.

Until he tried a marijuana look-alike product called “K2,” David Rozga’s most dubious decision was getting a Green Bay Packers tattoo on his shoulder.

Then the 18-year-old athlete and band standout got high on the fake pot last June and complained to a friend “that he felt like he was in hell,” his father said.

Though he had never suffered from depression, the teenager went home, found a shotgun and killed himself — one of at least nine U.S. deaths in the last year that authorities suspect were caused by synthetic products designed to mimic marijuana, cocaine and other illegal drugs.

An Associated Press analysis shows that the substances are increasingly causing users to fall seriously ill, with some suffering seizures and hallucinations.

Available in many head shops for as little as $10, the synthetic drugs are often packaged as incense or bath salts, but they do nothing to perfume the air or soften water.

As more Americans experiment with them, the results are becoming evident at hospitals: a sharp spike in the number of users who show up with problems ranging from labored breathing and rapid heartbeats to extreme paranoia and delusions. The symptoms can persist for days.

“These kids weren’t looking for anything bad to happen,” Mike Rozga said of his son’s death. “The truth is they didn’t know what they had gotten themselves into.”

At the request of the AP, the American Association of Poison Control Centers analyzed nationwide figures on calls related to synthetic drugs. The findings showed an alarming increase in the number of people seeking medical attention.

At least 2,700 people have fallen ill since January, compared with fewer than 3,200 cases in all of 2010. At that pace, medical emergencies related to synthetic drugs could go up nearly fivefold by the end of the year.

“Many of the users describe extreme paranoia,” said Dr. Mark Ryan, director of the Louisiana Poison Center. “The recurring theme is monsters, demons and aliens. A lot of them had suicidal thoughts.”

The recent surge in activity has not gone unnoticed by law enforcement and elected officials.

The Drug Enforcement Administration recently used emergency powers to outlaw five chemicals found in synthetic pot, placing them in the same category as heroin and cocaine.

But manufacturers are quick to adapt, often cranking out new formulas that are only a single molecule apart from the illegal ones.

On Wednesday, the Senate’s Caucus on International Narcotics Control held a hearing in Washington to discuss curbing the growth of synthetics.

“This is a whole new method of trafficking,” testified Joseph T. Ranznazzisi, deputy assistant administrator in the DEA’s office of diversion control. “We’ve never experienced this before, when the product is just on the shelf.”

Rozga implored lawmakers to act swiftly to prevent more deaths: “We are not doing enough, and we are not moving quickly enough.”

Recreational drugs created in the laboratory have been around at least since the middle of the 20th century, when LSD was first studied. But these latest examples emerged only a few years ago, starting in Europe.

The products were typically made in China, India and other Asian nations and soon arrived in Britain and Germany, according to DEA spokesman Rusty Payne.

In the United States, fake marijuana was last year’s big seller, marketed under brands such as “K2” or “Spice.” This year, the trend is “bath salts” with names like “Purple Wave” and “Bliss.”

Besides being cheap and easily obtained, they do not show up in common drug tests.

Synthetic marijuana typically involves dried plant material sprayed with one of several chemical compounds, most of which were created by a Clemson University scientist for research purposes in the 1990s. The compounds were never tested on humans.

It’s packaged to look like pot, and users typically smoke it, but experts say the high is more comparable to cocaine or LSD.