Signs of Concern

Someone under the influence of opioids will have tiny pupils.
Someone on a stimulant will have dilated pupils that are so large only a small portion of their iris will be visible.
People using marijuana will have eyes that look red and glassy.

Someone using any drug intravenously will have bruising and needle marks on their arms or other locations on their body.
Someone injecting anabolic steroids can have these marks in areas of large muscles like the upper thigh.
People abusing crystal meth can develop sores on their skin from scratching.
Crack users may show burn marks on their mouth and fingers.
Substances that damage the liver will result in skin that looks yellow.

Someone on a stimulant may spend days awake and constantly active, which will be followed by a crash where they sleep for days.
People abusing depressants may seem to be stuck between awake and asleep for long periods or seem to drift in an out of sleep.

Most substances result in a pleasurable, euphoric feeling initially, which encourages further use.
After the positive feelings subside, unwanted feelings will present. Increased depression, anxiety, irritability, anger, and irrationality are common among substances abusers.

Obsessive behaviors. Staying up all night to do iron or repeating the same task or phrase over and over in a compulsive manner can be a sign of certain drugs like meth.
Missing for long periods of time and establishing new routines with new friends are common behavioral signs.
Secretiveness. A shift in communication style to be more guarded can mean the person is trying to hide substance use.
Drastic change in beliefs. If someone was previously very religious and focused on their family but now has time for neither, drug abuse might be a factor. Or, someone who was previously not very religious may develop preoccupations with religious content (e.g., quoting or reading the bible persistently).
Using measures to conceal. Abusers may try to hide signs of abuse, for example by wearing sunglass inside or long-sleeves in the summer.
Paraphernalia. Finding pill bottles, large amounts of money, empty alcohol containers, needles, scales, and other paraphilia hidden around the house are all warning signs.
Random injuries and illness. While intoxicated, people are more likely to become injured. If someone is having more "accidents," drugs could be the culprit.

People using marijuana may gain weight due to decreased motivation and activity levels.
Most notably, stimulants can reduce the appetite to a point where people stop eating completely. Any significant, unintentional weight change can point towards abuse.