Hello!
Alcohol and drugs can cloud a person's judgement and awareness of what is happening around him. He will likely make wrong decisions because he will not be able to understand the decision clearly while under the influence.
Drug and alcohol use can affect a person's life because it can change his reputation. Drug use can instantaneously make a person ineligible for certain jobs and positions, which can greatly impact a person's life.
I hope this helps answer your question! Have a great day!