How can the use of alcohol or other drugs increase a person's risk of unintentional injury?

Because both impair a person's motor functions so they end up more clumsy and don't have any sort of filter to make them stop making bad decisions. 

Ingesting these both causes the parts of your brain with coordination and judgement to slow down. This impacts your views on reality and could make it so that you make poor decisions. This includes sociability, rowdiness, mood swings, etc.

Afew buddies come over to your house to hang out. they notice you're struggling in school, perhaps you have to rely on to pass your class, but you can't absorb the knowledge. they could positively reinforce getting a tutor and working with you, dedicating more time to the areas of struggle and less on hanging out to better yourself.secondly, say you're the victim of bullying. you're afraid to tell an adult because of possible repercussions from the bully himself. your friends encouraging you and pushing you towards telling an adult for your safety is another example.