You keep taking a drug after it's no longer needed for a health problem.
You need more and more of a substance to get the same effects (called "tolerance"), and you can take more before you feel an effect.
You feel strange when the drug wears off. You may be shaky, depressed, sick to your stomach, sweat, or have headaches. You may also be tired or not hungry. In severe cases, you could even be confused, have seizures, or run a fever.
You can't stop yourself from using the drug, even if you want to. You are still using it even though it's making bad things happen in your life, like trouble with friends, family, work, or the law.
You spend a lot of your time thinking about the drug: how to get more, when you'll take it, how good you feel, or how bad you feel afterward.
You have a hard time giving yourself limits. You might say you'll only use "so much" but then can't stop and end up using twice that amount. Or you use it more often than you meant to.
You've lost interest in things you once liked to do.
You've begun having trouble doing normal daily things, like cooking or working.

You drive or do other dangerous things (like use heavy machines) when you are on the drug.
You borrow or steal money to pay for drugs.
You hide the drug use or the effect it is having on you from others.
You're having trouble getting along with co-workers, teachers, friends, or family members. They complain more about how you act or how you've changed.
You sleep too much or too little, compared with how you used to. Or you eat a lot more or a lot less than before.
You look different. You may have bloodshot eyes, bad breath, shakes or tremors, frequent bloody noses, or you may have gained or lost weight.
You have a new set of friends with whom you do drugs and go to different places to use the drugs.
You go to more than one doctor to get prescriptions for the same drug or problem.
You look in other people's medicine cabinets for drugs to take.
You take prescribed medications with alcohol or other drugs. 

Appearing intoxicated more and more often and experiencing withdrawal symptoms when they are unable to take the drug
Being lethargic, sleeping more, sleeping irregular hours, or appearing unwell or tired
Shakes, tremors, loss of memory, bloodshot eyes, frequent bloody noses, or slurred speech
Change in their daily routines
Lying about the substance or how much they are using, and becoming angry, sad, or lashing out when questioned about their substance abuse
Changes in personality and behavior like a lack of motivation, irritability, and agitation
Lack of concern for personal hygiene
Unusual need for money, financial problems, or stealing money or valuables to pay for drugs
Developing problems at work or school; possibly losing one’s job or dropping out of school