Starbucks coffee averages 56 percent more caffeine than coffee purchased at gas stations or doughnut shops, according to a study commissioned by The Wall Street Journal.

Slideshow: Starbucks Tops Caffeine Study

The Journal hired a laboratory to compare the most widely available ready-made coffees with samples from 7-11 and Dunkin' Donuts nationwide.

The study found that house blends at Starbucks, Gloria Jean's and other gourmet-coffee chains have an average of 56 percent more caffeine than samples from 7-11, and 29 percent more than Dunkin' Donuts.

The Journal went on to wonder whether customer loyalty to such coffee houses came from the strong taste of the coffee, or its habit-forming effects.

The Journal said research showed people often choose to maintain their coffee intake not for its positive effects, but more to avoid the irritable effects when they don't get it.

The Journal noted that in recent years, researchers have quantified specific doses of caffeine and linked them with such withdrawal symptoms as headaches, drowsiness and difficulty concentrating.

Caffeine withdrawal usually begins within 12 to 24 hours after the last sip, experts said, which may explain why people often reach for their mug first thing in the morning.