Researchers, part of a World Health Organization research consortium at Harvard Medical School in Boston, said people who have ADHD find it difficult to concentrate because they may be hyperactive, easily distracted, forgetful or impulsive. Children are being increasingly diagnosed because they are likely to be tested for ADHD if they have problems with their schoolwork; however, many adults with ADHD do not know they have the condition.

The study of more than 7,000 employed and self-employed workers in several countries ages 18 to 44 were screened for ADHD as part of the World Health Organization World Mental Health Survey Initiative. They were also asked about their performance at work in the last month.

The study, published online ahead of print in Occupational and Environmental Medicine, said people with ADHD were found to spend 22.1 more days not doing work than other workers per year -- 8.4 days when they were unable to work or carry out their normal activities, 21.7 days of reduced work quantity and 13.6 days of reduced work quality.

The researchers suggest adult ADHD might be a candidate for targeted for treatment programs.