It's not always easy to tell when someone you know and love is becoming a drug addict or alcoholic. They will tend to try to hide their problem, or outright deny it, so you need to look for telltale signs or changes in behavior. The biggest give-away is if the person starts getting in trouble with the law, or if he or she starts visiting doctors frequently without a clear reason; they're sure to have excuses, and you'll have to make a judgment call on how reasonable they are. A major personality change in an adult (over 24 years old) is either a sign of a serious physical or mental illness, or of substance abuse.
Other signs to look out for are more subtle, or may be indications of different problems; however, they're usually pointing to some situation that requires intervention and help. Changes in eating and sleeping habits can be caused by addiction. For instance, people using crystal meth often stay awake for days before crashing into days of sleep. They also tend to devour sugary foods. Significant weight loss or gain can be a side effect of addiction, as can a tendency to neglect personal grooming. If someone who usually keeps fit and healthy stops doing so, it's likely that something is wrong.
Family and social connections often suffer when someone is an addict. The person may cut contact with relatives and his or her usual friends. S/he may spend more time alone locked in the bedroom, in front of the computer or television, or hanging out with people s/he wouldn't previously have been friends with. It should also be remembered that children of alcoholics are more at risk of becoming alcoholics themselves.
When someone's performance at work or at school begins to deteriorate, this is another warning that something may be wrong. In all these cases, addiction or alcoholism are possible causes, and if you can uncover the problem and help the person get beyond it, you will have done a worthy deed.