Virtually every medication presents some risk of undesirable side effects, sometimes even serious ones. Doctors consider the potential benefits and risks to each patient before prescribing medications. They understand that drugs affect the body in many ways and take into account things like the patient’s age, weight, and medical history; the drug’s form, dose, and possible side effects; and the potential for addiction. People who abuse drugs might not understand how these factors interact and put them at risk, or that prescription drugs do more than cause a high, help them stay awake, help them relax, or relieve pain.

Personal data. Before prescribing a medication, doctors take into account a person's weight, how long they've been prescribed the medication, and what other medications they are taking. Someone abusing prescription drugs may overload their system or make themselves vulnerable to dangerous drug interactions that can cause seizures, coma, or even death.
Form and dose. Doctors know how long it takes for a pill or capsule to dissolve in the stomach, release drugs to the bloodstream, and reach the brain. When abused, prescription drugs may be taken in inappropriate doses or by routes of administration that change the way the drugs act in the body and brain, presenting overdose risk. For example, when people who abuse OxyContin crush and inhale the pills, a 12-hour dose hits their central nervous system all at once—which increases the risk of addiction and overdose.
Side effects. Prescription drugs are designed to treat a particular illness or condition, but they often have other effects on the body, some of which can be dangerous. These are referred to as side effects. For example, OxyContin stops pain, but it also causes constipation and drowsiness. Stimulants such as Adderall increase attention but also raise blood pressure and heart rate. These side effects can be worse when prescription drugs are not taken as prescribed or are abused in combination with other substances—including alcohol, other prescription drugs, and even over-the-counter drugs, such as cold medicines. For instance, some people mix alcohol and benzodiazepines (e.g., Valium), both of which can slow breathing. This combination could stop breathing altogether.
Addiction. Studies show that when people take a medication as it is prescribed for a medical condition—such as pain or attention deficit hyperactivity disorder (ADHD)—they usually do not become addicted, because the medication is prescribed in dosages and forms that are considered safe for that person. The person is also monitored by a physician. The drug addresses a real problem, which makes the person feel better, not high. But medications that affect the brain can change the way it functions—especially when they are taken repeatedly or in large doses. They can alter the reward system, making it harder for a person to feel good without the drug and possibly leading to intense cravings, which make it hard to stop using. This is no different from what can happen when someone takes illicit drugs—addiction is a real possibility.