Even though teen drug use is on the decline, it is still a major issue and one that parents are right to worry about. According to the latest Monitoring the Future survey, which has looked at trends in teen drug use since 1975, around 9.8% of 10th graders and 14.3% of 12th graders do use or have used drugs.

Parents can play a key role in helping to reduce these numbers, especially if they understand what to look for in their teen and are aware of the top signs of teen drug use. Medline notes that these can include: worsening grades, school absences, mood swings, and physical changes such as slurred or rapid speech, weight and appetite changes, and sluggish or hyperactive behavior. However, there are other lesser-known signs that your child may be using – it is also good to also be on the lookout for these more subtle signs as well.

Social Changes
It is not abnormal for teenagers to make new friends or change groups during the course of their high school careers. However, according to DrugAbuse.com changing or abandoning friends and running with a different crowd can be a sign that your child might be hanging out with people who abuse substances. Sober Nation notes that other social changes include dropping out of family events and gatherings and becoming, in general, more socially isolated.

Memory Problems
Not all memory loss is drug-related. It can be typical for many teens to be forgetful about things like homework, schedules, etc., or this behavior could be a sign of a medical problem like attention deficit disorder (ADD). However, if your child is forgetting basic information, it could be a sign that they are using.

Unexplained Injuries
Unexplained injuries are not always the first thing that comes to mind when parents think about drug abuse, but it can be a sign that a teen is getting injured while they are impaired. This is because, as Sober Nation notes, many drugs can decrease coordination, balance, and judgement and this can make injury more likely. Common injuries can include bruises, black eyes, and sprains, and teens will often give vague or conflicting stories to explain them away.

Frequent Illnesses
The side effects of some drugs include the suppression of the immune system. If your teen seems to be getting sick more often than normal and appears to be more vulnerable to infections that are making the rounds, it could be because of drug use. Teens will often report vague illnesses and feeling “under the weather” when they are using and sometimes use it as an excuse to get out of school, work, and other functions.

In short, the signs and symptoms of teen drug use are not always obvious. It is important for parents to know that substance misuse can affect different teens in different ways and that the signs to look for are not always obvious. This knowledge can be invaluable for parents to become aware of their teen’s drug problem, which is the first step to getting needed help.

Brian Wu is a current PhD graduate and MD candidate who enjoys writing about health, science and the ways it can “hack” your life. He is currently working on Health Stories for Kids — Medical Ebooks that Educate and Entertain. You can follow Brian on Twitter.