"They don't want to pay whatever those consequences would be," said Stacie Allphin, director of adolescent programs at Memorial Hermann Prevention and Recovery Center in Houston.
She said that teenagers may also be dishonest about their drug use because of suspicion that their parents or authorities will find out.
"With cocaine especially, there's a lot of stigma around that, especially if the cocaine type is crack cocaine. People don't want to be associated as crack addicts, so kids would tend to underreport [using cocaine]."
The study found that teens reported they used marijuana more frequently than their hair samples showed. The researchers say that's likely because the biological testing wasn't sensitive enough. Other experts say it could be due to an entirely different reason.
"It also may be a common belief that marijuana is not as serious a drug, and there's a wider social acceptability of it," said Victoria Talwar, associate professor in the Department of Educational and Counseling Psychology at McGill University in Montreal.
But drug use is just one thing teens often lie about.
"The most common thing is something they've done that they think their parents will be upset about," said Talwar. "That can be a range of things."
She said that's because they don't want their parents to see them as bad kids, and they don't want to deal with lectures or other negative consequences that may result from getting caught doing something they shouldn't be doing.
For most young people, lying is just part of growing up.
"Teenagers often tell lies to exert some control over their lives and self-identify," said Talwar. "They're trying to assert their identities and use strategies to do that."
Although the study focused on urban, African-American teens, Talwar said that sex, race, socioeconomic status and other factors have no bearing on lying.
"Lying is part of human nature, and people lie for universal reasons, regardless of population."
Regardless of why teens may not be truthful about their drug use, experts say it's important to realize that it could be more common than studies suggest.
Said Allphin: "When they come into treatment, they may be experiencing withdrawal symptoms that need to have medical attention. The problem with underreporting there is, it's life-threatening."