American consumers of cigarettes and other tobacco products have an increasing tendency to also consume marijuana, according to new findings from an American research group. Marijuana and cigarettes are two of America\u2019s three most commonly consumed addictive substances. While marijuana use is on the rise, cigarette use has fallen consistently for a number of years. In a study scheduled for publication in October 2015 in the journal Addictive Behaviors, researchers from Emory University used data gathered from hundreds of thousands of American adults to assess the frequency of marijuana use among people who smoke cigarettes or use other tobacco products. These researchers concluded that the number of marijuana-smoking tobacco consumers has risen considerably in the 21st century. Cigarette Use in America Since 2003, the percentage of Americans who smoke cigarettes at least once a month has fallen from roughly 26 percent to just over 21 percent. Crucially, the smoking rate among adolescents between the ages of 12 and 17 (a group particularly susceptible to the onset of nicotine addiction) has fallen by more than 50 percent over the same span of time. People in three age groups are substantially more likely to smoke: young adults in their early to mid-20s, young adults in their late 20s and adults in their early 30s. Teenage boys and teenage girls smoke with more or less equal frequency; however, men smoke substantially more often than women. Marijuana Use in America About 7.5 percent of all Americans over the age of 11 use marijuana one or more times per month, the federal Substance Abuse and Mental Health Services Administration notes in its annual National Survey on Drug Use and Health. This is the equivalent of 19.8 million individuals. Throughout most of the 2000s, the annual rate of marijuana intake shifted up and down, with a low point of 5.8 percent and a high point of 6.2 percent. However, in 2009, the rate embarked on a slow but steady rise that has continued over time. Marijuana is consumed more often than any other substance that meets the federal standard for an illicit or illegal drug or medication. As a rule, peak usage rates for the drug appear among young adults in their early to mid-20s. Cannabis Addiction and Nicotine Addiction Both marijuana and cigarettes contain substances capable of making long-term alterations in the daily function of the brain\u2019s pleasure center, and thereby establishing the requirements for the onset of physical dependence and addiction. In marijuana and other forms of cannabis, the key addiction-related chemical is THC (tetrahydrocannabinol). In cigarettes and other tobacco products, the key chemical is nicotine. Anywhere from one-quarter to one-half of all habitual marijuana consumers will develop a diagnosable case of cannabis addiction sooner or later. Over half of all adult smokers have a nicotine addiction. Frequency of Overlapping Use In the study scheduled for publication in Addictive Behaviors, the Emory University researchers used data from 10 years of the National Survey on Drug Use and Health (2003 to 2012) to help determine how often American adults who consume cigarettes or some other tobacco product also consume marijuana, or vice versa. All told, the researchers included information drawn from 378,459 adults. After analyzing the survey results, they found that just over 5 percent of all U.S. adults used both marijuana and cigarettes or some other form of tobacco in the final year under consideration (2012). In that year, 24 percent of American adults used tobacco but did not smoke marijuana; in addition, 2.3 percent of adults smoked marijuana but did not use tobacco. The researchers concluded that the combined use of marijuana and cigarettes\/tobacco rose among American adults between 2003 and 2012. Interestingly, tobacco use fell noticeably in people who primarily smoked marijuana, while marijuana use increased noticeably in people who primarily consumed cigarettes\/tobacco. Such a trend is consistent with the overall increasing popularity of marijuana and the overall decreasing popularity of cigarettes. The study\u2019s authors note that the combined use of marijuana and tobacco did not rise in 18- to 25-year-olds, a group with relatively heavy involvement in both marijuana intake and cigarette intake. However, combined use did increase in several other demographic groups, including men, women, adults in their late 20s and early 30s, adults 50 and older, Caucasian Americans, African Americans and Hispanics.