Marti Patterson of Olympia was thrilled when she discovered a cranberry energy drink called "Cranergy" on store shelves.

"Oh, it's delicious! It wasn't too sweet, it was satisfying," said Patterson.

She chose the Ocean Spray beverage because it was only 50 calories per 12 ounce serving and she loved the taste.

And Patterson didn't think it had any caffeine because the label doesn't list it.

KIRO 7 Consumer Investigator Amy Clancy: "Do you drink caffeine usually?"

Patterson: "No."

Clancy: "None at all?"

Patterson: No, I drink decaf coffee."

Clancy: "So you don't even have a cup of coffee in the morning?"

Patterson: "No, and I drink decaf beverages."

Which is what Patterson thought she was drinking.

"I would drink two during a normal day, maybe. And if I exercised I would drink one on the way to the gym and one on the way back, just for the fluid," said Patterson.

Within two weeks of often drinking four bottles of Cranergy a day, Patterson says she wasn't sleeping or feeling well.

Patterson: "I started feeling worse and worse, and by 11 a.m. I went to the refrigerator to look at the problems for heart attacks because I was having chest pains, I was nauseated, I was perspiring profusely and what go my attention was my left shoulder and my left arm were hurting."

Clancy: "So what did you think was happening?"

Patterson: "I thought I was having a heart attack."

Patterson drove herself to the emergency room at Providence St. Peter's Hospital in Olympia, where she was examined for possible heart problems, given nitroglycerin and even a stress test.

"When I started to have the stress test, their question was, 'Have you had caffeine since breakfast yesterday morning?' and I said, 'I have been drinking a beverage that has green tea extract in it, does that have caffeine in it?' and the nurse said, 'Oh, yes,'" said Patterson.

KIRO 7 asked St. Peter's hospital for an interview with the doctors and nurses who treated Patterson.

That request was denied.

But Patterson says when it was confirmed that she did not have a heart attack, she asked her doctor if the caffeine in Cranergy could have caused whatever it was she was experiencing.

"He said, 'Absolutely. Stop drinking it,' and I said, 'Thank you very much.' After I stopped drinking the tea, I started sleeping and I started feeling better than I had in a long time," said Patterson.

It's impossible to know for certain whether the natural caffeine in Cranergy's green tea extract caused Patterson's problems, but the effects of caffeine, both positive and negative, have been widely studied.

According to the Food and Drug Administration, "there are clearly people who have serious cardiac arrhythmias (irregular heartbeat) due to caffeine consumption."

A recent study by the Journal of Analytical Toxicology reveals, "because of health concerns arising from the consumption of caffeine, it seems appropriate that warning labels should accompany all caffeinated beverages."

In the European Union, drinks with more than 150 milligrams of caffeine per liter must have a "high caffeine content" label.

If the same standard applied in the U.S., Cranergy, with roughly 216 milligrams of caffeine per liter, would have to include the same warning label.

"I was just overwhelmed by how concentrated the green tea extract actually made the caffeine levels," said Seattle University senior Valeria Maxey.

A chemistry class at Seattle University, taught by professor Susan Jackels, recently studied caffeine in beverages.

"We found out that the Cranergy has a significant amount of caffeine in it," said Jackels.

"It's right below Red Bull. Barely below. Red Bull has about 80 milligrams per serving. Cranergy has 76," said Seattle University sophomore Brien Birge.

KIRO 7's Amy Clancy asked Ocean Spray how a consumer would know about the caffeine in Cranergy since it's not listed on the label?

In an e-mail, a spokeswoman wrote, "Consumers can get information via the 800 number provided on the bottle."

The e-mail also points out that an 8 ounce serving of coffee has 95 milligrams of caffeine.

By comparison, an 8 ounce serving of Cranergy has 70.

But the e-mail does not reveal that Cranergy is sold in 12 ounce bottles, for a total of more than 100 milligrams of caffeine.

"It's about the same as a cup of coffee," said Birge.

The Seattle University students and their professor agree with the American Medical Association and the Center for Science in the Public Interest, which have been lobbying unsuccessfully since 1997 for the FDA to require that food and beverage companies clearly label caffeine content.

"Voluntary labeling ought to be replaced by mandatory labeling," said Jackels.

"In this country, we don't put warnings on things that have too much caffeine in them, or else Rockstar and Red Bull would have a warning on it. But this drink is a little more subversive I think because it tried to market itself as if it were a natural health drink, and it's clearly no better than a Red Bull or a Rockstar," said Seattle University senior Kenneth Langley.

Last year, Pepsi and Coca Cola started voluntarily disclosing caffeine content on their labels.

Now, many other national beverage brands also do the same.

But Ocean Spray refused to reveal whether it would consider changing its labels.

In fact, when asked by Clancy for more information, Ocean Spray's only other comment was, "no comment." Others, however, have plenty to say.

"I think it's almost irresponsible that it's labeled the way that it is," said Seatle Univiersity student Chris Shreeve.

"I think I have a right to know what I'm putting into my body," said Aziz Atalah, a sophomore at Seattle University.

Patterson: I just want them to let people know what is in their drink. Just disclose the caffeine."

Clancy asked the Food and Drug Administration if it might rethink its caffeine labeling guidelines.

A spokesperson told her that at this point, "due to competing agency priorities and the limited availability of resources," there's no plan to do so.

The bottom line, according to the FDA, is that if caffeine is added, it should be included in the ingredient list. If it's naturally present, as it is in green tea extract, it doesn't have to be on the label.

Cranergy does have plenty of fans, and is generating a lot of positive buzz online. One blogger wrote, "Love the Cranergy drink! I really can't have the caffeine drinks, and I find this drink gives me just the boost I need," the blogger said.