Eight years after a multi-billion-dollar master settlement between tobacco companies and 46 states on the health problems linked to smoking, cigarettes now have more nicotine—the substance that makes them addictive.

Tobacco companies have raised nicotine levels in cigarettes by 11 percent—about 1.6 percent per year—between 1998 and 2005, researchers at the Harvard School of Public Health (HSPH) confirmed on Thursday.

The findings were first reported by the Massachusetts Department of Public Health and later turned over to researchers at the Harvard School of Public Health for independent analysis.

“Cigarettes are finely-tuned drug delivery devices, designed to perpetuate a tobacco pandemic,” one of the study’s leaders, Howard Koh, associate dean for public health practice at HSPH, said in a statement.

Researchers found that the increase held across the board, without regard to cigarette type or manufacturer, and that these increases occurred without any warning to consumers.

Tobacco companies used a variety of methods including design modification and increased concentrations of nicotine, according to the study which analyzed nicotine levels in cigarettes from Phillip Morris USA, Reynolds American and Lorillard Tobacco. Cigarette manufacturers are required by the state of Massachusetts to disclose nicotine yields each year.

HSPH professor Gregory Connolly, another principal of the study, said in a statement, “our findings call into serious question whether the tobacco industry has changed at all in its pursuit of addicting smokers since signing the Master Settlement Agreement of 1998 with the State Attorneys General.”

However, the study’s leaders were careful to note that lower nicotine content in cigarettes may not affect overall nicotine intake.

Previous studies have shown that smokers tend to take “larger puffs” or smoke more cigarettes if they smoke cigarettes with less nicotine in them, the study’s authors said.

But in a statement on their Web site, Phillip Morris USA challenged the Harvard study by claiming that “nicotine yields for Marlboro cigarettes were the same in 2006 as they were in 1997.”

It also attributed any reported changes to “random variations in cigarette nicotine yields, both upwards and downwards.”

But the researchers said that the increase in nicotine yield over the period studied was statistically significant and not due to random variation.

The U.S. Supreme Court ruled in Jun. 2000 that the Food and Drug Administration (FDA) does not have the authority to regulate the tobacco industry, including nicotine levels.

But Sen. Edward Kennedy ’54 (D-Mass.), incoming chairman of the Senate’s Health, Education, Labor and Pensions Committee, indicated last week that he plans to introduce legislation to put the industry under FDA regulation.

“This study is an extraordinary public service by Harvard’s School of Public Health,” said Kennedy in a statement on his Web site.

More than 440,000 Americans die each year from diseases related to tobacco use, according to the American Heart Association.