LOS ANGELES — Despite the now well-known and far-reaching effects of smoking, more than one-fifth of Americans still smoke. Why?

For many, it may be hard to quit because they also have depression. According to government statistics released Tuesday, 43 percent of adult smokers age 20 and older have depression. Among men ages 40 to 54, a whopping 55 percent of those who smoke have depression. Among women ages 20 to 39 who smoke, 50 percent have depression.

The report, drawn from the National Health and Nutrition Examination Surveys of 2005 through 2008, also found depressed smokers are heavier smokers. Depressed people were more likely to smoke within five minutes of awakening and to smoke more than one pack of cigarettes a day. Twenty-eight percent of adult smokers with depression smoked more than a pack a day, which is almost twice the rate for adult smokers without depression.

Smokers with depression were also much less likely to have tried quitting. However, other studies have demonstrated that smokers with depression can succeed with intensive treatment. These treatments usually include therapies for help with depression and smoking cessation.

The report was published by the National Center for Health Statistics. Information on smoking cessation can be found online at www.smokefree.gov.