Is ampicillin safe for a 14 yr old?

Yes: As long as he or she is not allergic to penicillin.

Yes,but: What is your concern. A prescription medication is safe if used as directed for the condition it is prescribed for when the patient is not allergic to that medication.It would not be considered safe if a few left over capsules were given because someone thought it might help a problem.Such random use has delayed patients from accessing proper diagnosis & treatment, occasionally with tragic results.