She admits that during a tough week at Boston University, it wouldn’t be unusual to find three or four empty cans around her room. But junior Nikki Thalheimer says she will never let herself get to the point where her body craves a drink.

“But when it gets to finals time, I probably have four a day for like, two weeks,” she says.

Thalheimer’s substance of choice is caffeine – usually in the form of a Starbucks’ Double Shot.

According to research done at John Hopkins University School of Medicine, caffeine is the most widely used mood-altering drug in the world – consumed habitually by between 80 and 90 percent of adults and children in the United States.

Caffeine is most commonly consumed in the form of coffee, tea or soft drinks but is prevalent in many forms, including chocolate, pain relievers and cold remedies. Medically, caffeine is a stimulant in the same family as medications prescribed for illnesses like Attention Deficit Disorder, and enhances the effects of pain medication.

Dr. David Newman, who owned a private practice in internal medicine in Ithaca, N.Y. for 10 years and has worked at Ithaca College for 13 years, says caffeine is a mild stimulant that works in the body the same way natural adrenaline does. Consuming caffeine is proven to improve mental function but can also have adverse side effects when taken in excess.

Caleb Knodell, a sophomore at Missouri State University, says he takes in about 200 milligrams of caffeine a day – which he says is a considerable decrease from the amount he used to consume.

“My addiction started mainly because I loved soda,” he says. “I’d drink soda at least six to seven times a day, for every meal, like water.”

Knodell says he actively tried to wean himself off caffeine when he considered the effects it was having on his body.

“I was putting my body in a mode that it was unhealthy to be in the majority of the time,” he says.

But Newman says there are no proven medical problems with drinking between one and four cups of coffee a day, which he says is probably the average. Since one cup of brewed coffee contains about 100 milligrams of caffeine, that’s between 100 and 400 milligrams each day.

“If someone was using caffeine to the point where they were suffering ill effects from it and yet they were continuing to use it I’d be worried,” he says. “And I think most physicians would sort of raise their eyebrows a little at hearing someone using really dramatically more caffeine than the average person.”

But even as little as 100 milligrams of caffeine per day can cause dependency and withdrawal symptoms including headache, fatigue, irritability and nausea.

Thalheimer says she doesn’t typically experience any side effects of her habit but once developed a twitch in her eye that only subsided after three weeks without caffeine. She says after that she was more careful about not becoming dependent.

“Sometimes I get to the point where I’m like, Oh, I need coffee now,” she says. “I try to restrict myself when I feel like I have a need for it.”