THURSDAY, Jan. 18 (HealthDay News) -- U.S. tobacco companies increased the level of addictive nicotine in their cigarettes by 11 percent from 1998 to 2005, and did so in a variety of ways, new research shows.

The tobacco companies accomplished the increase not only be intensifying the concentration of nicotine in the tobacco but also by modifying several design features of cigarettes to increase the number of puffs per cigarette taken by smokers, according to a Harvard School of Public Health (HSPH) study.

Analyzing major brand name cigarettes sold in Massachusetts, the researchers found that increases in smoke nicotine yield per cigarette averaged 1.6 percent each year over those seven years. Nicotine is the primary addictive ingredient in cigarettes.

Increases in smoke-nicotine yields occurred in each of the four major manufacturers and across all the major cigarette market categories, including light and ultralight.

The findings were expected to be presented Thursday at the Harvard School of Public Health.

"Cigarettes are finely-tuned delivery devices, designed to perpetuate a tobacco pandemic," research team co-leader Howard Koh, associate dean for public health practice at HSPH and a former commissioner of public health in Massachusetts, said in a prepared statement.

"Yet precise information about these products remains shrouded in secrecy, hidden from the public. Policy actions today requiring the tobacco industry to disclose critical information about nicotine and product design could protect the next generation from the tragedy of addiction," Koh said.

"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," research team co-leader Gregory Connolly, program director of the Tobacco Control Research Program at HSPH, said in a prepared statement.

"Our analysis shows that the companies have been subtly increasing the drug nicotine year by year in their cigarettes, without any warning to consumers, since the settlement. Scrutiny by the Attorneys General is imperative," Connolly said.

He added that Sen. Edward Kennedy (D-Mass.) has filed proposed federal legislation "that would address this abuse and bring the tobacco industry under the rules that regulate other manufacturers of drugs."

More information

The U.S. National Institute on Drug Abuse has more about tobacco addiction.