A new study finds some brands of cigarettes have increased the amount of nicotine they deliver. This is likely to make them more addictive, according to the researchers.

They studied data provided to the Massachusetts Department of Public Health by four major cigarette companies from 1997 to 2012, as required by state law. They found while the average amount of nicotine in cigarettes has remained fairly stable in recent years, the average amount of nicotine delivered from smoking a cigarette increased from 1.65 milligrams in 1999, to 1.89 milligrams in 2011.

This 15 percent increase could have been the result of a redesign of some brands of cigarettes, The Boston Globe reports. Lead researcher Thomas Land said cigarette makers could have changed the filter or length of their products to increase the efficiency of nicotine delivery.

The researchers used smoking machines to study the amount of nicotine delivered by various brands of cigarettes. They found RJ Reynolds, B&W (now a part of RJR) and Philip Morris produced brands with higher nicotine yields in 2012, compared with 2005. Lorillard produced cigarettes with a lower nicotine yield.

The findings appear in Nicotine and Tobacco Research.

The Food and Drug Administration does not set limits on the amount of nicotine allowed in cigarettes, or on a maximum amount of nicotine yield per cigarette.

“This study indicates that cigarette manufacturers have recently altered the design of cigarettes. This can significantly increase the amount of nicotine a person receives while smoking,” Land said in a news release. “Cigarettes have a more efficient nicotine delivery system than ever before. Because smokers have no way of knowing that the level of nicotine they are receiving has increased, they can become more addicted more easily without knowing why.”