For starters: you should never be worried about overreacting when it comes to things like this. Alcoholism and addiction are dangerous diseases that can be deadly.
To strictly answer your question: regularly drinking turns to addiction kind of subtly; there are stages to alcoholism. It begins with a few drinks here or there and then gradually progresses. It turns into a full-fledged addiction when drinking becomes more of a need in order to function semi-normally rather than a wan. Other factors include deteriorating relationships, making excuses, trouble sleeping, or experiencing withdrawal symptoms if they don't drink.
Any stage of alcoholism is troubling, especially if the person denies there is anything wrong.
Approach them in a loving way and let them know that you are worried. Talk with them without blame or accusations and let them know that you are worried. Don't be afraid to reach out to a professional who specializes in addiction and intervention and join a support group like Al-Anon, a group for loved ones of alcoholics. The truth is, if you're already worried, then there is already a problem.