What does drug misuse mean?

This answer is multifaceted:

1) Taking a drug that was not prescribed to you.

2) Taking a drug for a reason other than what it was prescribed for.

3) Taking more than the prescribed/recommended dose.

4) Allowing someone else to take your drug.

5) Selling your prescription drug to someone else.

The list can go on and on.... it can be intentional(lying about pain levels to get more narcotic) or unintentional (an elderly person taking more than the prescribed amount due to visual impairment.