There are many different indicators of drug use. These vary depending on the quanity and type of drug used. Here are some basic signs.

Physical:

Marijuana: Glassy, red eyes; dilated (large) pupils;  loud talking and inappropriate laughter followed by sleepiness; a sweet burnt scent; loss of interest, motivation; weight gain or loss.

Alcohol: Clumsiness; difficulty walking; slurred speech; sleepiness; poor judgment; dilated pupils.

Depressants: (including Benzodiazepines and tranquilizers) Seems drunk as if from alcohol but without the associated odor of alcohol; difficulty concentrating; clumsiness; poor judgment; slurred speech; sleepiness; and contracted pupils.

Stimulants (i.e. Cocaine, Meth): Hyperactivity; euphoria; irritability; anxiety; excessive talking followed by depression or excessive sleeping at odd times; may go long periods of time without eating or sleeping; dilated pupils; weight loss; dry mouth and nose.

Inhalants: (Glues, aerosols, and vapors ) Watery eyes; impaired vision, memory and thought; secretions from the nose or rashes around the nose and mouth; sores on the tounge and mouth from freezing or chemical burns;  headaches and nausea; appearance of intoxication; drowsiness; poor muscle control; changes in appetite; anxiety; irritability; an unusual number of spray cans in the trash.

Hallucinogens: Dilated pupils; bizarre and irrational behavior including paranoia, aggression, hallucinations; mood swings; detachment from people; absorption with self or other objects, slurred speech; confusion.

Opiates (Heroin, Oxycontin, Vicodin): Needle marks; sleeping at unusual times, being unable to keep eyes open; sweating; vomiting; coughing and sniffling; twitching; loss of appetite; contracted pupils; no response of pupils to light; itching.

Behavioral changes with all drugs:

Missing cash/valuables, stealing/borrowing money; Difficulty in maintaining employment; Hostility towards others; Broken commitments; Loss of interest in usual, healthy activities; Unexplained absences at work, school or family events; Change in performance, academic or otherwise; Change in eating or sleeping patterns.