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Smoking a drug can cause red or watery eyes. Pupils that are constricted or dilated can be a sign of narcotics, stimulants, or club drugs. Check for rapid or unnecessary eye movements. Involuntary eye movements, or nystagmus, are symptomatic of many kinds of drug use. If someone is wearing sunglasses inside or in the shade, they might be trying to hide red or otherwise affected eyes. Someone who has smoked marijuana might smell sweet, smoky, or skunky. A chemical or metallic smell might mean they inhaled a toxic household product, such as glue or paint thinner. A smell of incense, air freshener, or powerful perfume or cologne might be intended to cover up the smell of a drug that has been smoked. Listen to his swallowing and observe the way it moves. Salivation and lip smacking can be signs of dry mouth, a sign of drug use. Licking of the lips, frequent clenching of the teeth or twisting of the jaw might mean that someone is high on club drugs. A bloody nose with no other apparent cause can mean that someone has snorted a drug, such as cocaine, meth, or a narcotic. A runny or congested nose can have many causes but combined with other symptoms it could signify that someone is high. Frequent rubbing of the nose can also be a sign. Someone who has snorted a drug might have powder on her nostrils or upper lip. Shaking hands can be a sign of club drugs, inhalants, or hallucinogens. Palm sweat can be a sign of intoxication. Burned fingertips can be a sign that crack cocaine was smoked. Pulse, rate of respiration, temperature, and blood pressure can all be affected by drug use. If you feel safe touching the person in question, take his pulse and check his temperature. Cold, sweaty skin is one sign of drug use. An increase or decrease in blood pressure, an increased heart rate, or slowed breathing can all be signs of drug use. Some drugs can cause chest pain, and even heart attacks. Seek medical help for someone who seems to be experiencing pain in his chest. People who use drugs such as methamphetamine, bath salts, or heroin often inject their drugs, which leaves track marks. Check for darkened veins, lesions, and bruising around the veins. Lesions that are open and in the process of healing can be signs of recent drug use. Sores or a rash on the mouth or nose can also be a sign of habitual drug use. While pipes, rolling papers, syringes and rubber tubing might be easily recognised as drug paraphernalia, the unwarranted presence of household objects can also indicate recent drug use. Bent spoons, eye droppers, and cotton balls may be indicative of narcotic use. Razors, handheld mirrors, and tiny spoons might indicate the use of stimulants. Pacifiers, candy necklaces, and lollipops may be used by people on club drugs, such as Ecstasy, that cause the jaw to clench.

Summary:
Look into the person's eyes. Smell the person. Observe the person's mouth. Observe the person's nose. Observe the person's hands. Check the person's vital signs. Check for signs of habitual drug use. Check for drug paraphernalia.