The definition of drug abuse is the habitual taking of addictive or illegal drugs. Some signs or symptoms you will experience or see in someone who is using drugs will be the use of medicines prescribed for other people, continue to take a drug or medication after it is no longer needed for a health problem, or feel strange when the drug or medication wears off. You may be or may see in someone else depression, shakiness or tremors, sickness to stomach, sweating, bloodshot eyes, bad breath, frequent nosebleeds, loss of weight, or headaches. Additional signs or symptoms that can be noticed are small pupils, constipation, nausea, sensitivity to pain, shallow breathing, slurred speech, drowsiness, confusion, dramatically changing moods, and social withdrawal.