They've given great advice. As for overdose, it depends on the person: weight for example. What is a perfect dose for an adult male would probably be too much for a preteen girl. It's important to talk to a doctor or poison control in case treatment is needed.
How much was taken? If this is a concern, the would warrant a phone call to their doctor to discuss with them. If you live in the US, you can also contact poison control with this question.
If you think you or someone you know have taken a higher dose than prescribed call the dr who wrote it asap and let them know so they can advise you/them what to do.