While texting allows instant contact with friends (it’s not uncommon for teens to send 100 text messages a day), excessive thumb and finger typing could be damaging. In fact, health expert Peter W. Johnson, PhD, with the University of Washington’s School of Public Health in Seattle, says texting-related injuries for everyone are similar to those associated with long-term computer use, such as thumb and joint irritation, forearm and shoulder pain, fatigue and aggravation in tendons that move the thumb.

But how much texting is too much?

“It’s really hard to say,” Johnson says. But he adds that computer users who self-report that they work more than four hours per day are more than twice as likely to develop musculo-skeletal problems than those who work four hours or less.

Frank Farley, PhD, a psychologist at Temple University in Philadelphia, notes that excessive texting can take time away from extracurricular and social activities, both of which are key to teen development.