Opioids are a derivative or synthetic version of opium, like heroin.

Such as: OxyContin, Codeine, Percodan, Fentanyl, Vicodin (hydrocodone), Methadone, Morphine, Percocet, Demoral.
How it’s taken: Ingested orally as a tablet or capsule or liquid form; crushed and snorted; or cooked so it can be injected intravenously (I.V.).
Occasional use: At first you may feel indifferent to physical or emotional pain, nauseous, and a little drowsy. You may be constipated later. If you take too much (overdose), your breathing may slow down and you could die.
Sustained use over time: Painkillers are highly addictive and tolerance can increase over time.
Most dangerous when used with: Alcohol, antihistamines, barbiturates, benzodiazepines.

Such as: Xanax, Valium, Librium, Klonopin, Ativan.
How it’s taken: Orally; or crushed and snorted.
Occasional use: You may feel calm and sleepy, with less tension, anxiety, or panic. This feeling will diminish over time as the body builds a tolerance to the substance.
Sustained use over time: Potential for addiction. Withdrawal can be lengthy, painful, and cause seizures, and should be medically supervised.
Most dangerous when used with: Alcohol, pain medications, some over-the-counter cold and allergy medications.

Stimulants

Such as: Ritalin, Concerta, Adderall, Focalin, Dexedrine.
How it’s taken: As a tablet; crushed and snorted; or liquefied so it can be injected intravenously (I.V.). Some people opt for inserting the drug anally (plugging).
Occasional use: You may feel alert, focused, and awake. Can make blood pressure and heart rate go up. Can suppress appetite and cause sleep problems.
Sustained use over time: High potential for addiction, paranoia, and sleep deprivation, which can cause psychotic episodes (like amphetamine psychosis). Also can cause insomnia, digestive problems, and erratic weight change.
Most dangerous when used with: Over-the-counter medications, including cold medications containing decongestants; antidepressants, unless supervised by a physician; some asthma medications.

This category includes a wide variety of substances. The similarity is that they are all cheap, widely available, and legal without a prescription.

Such as: Dextromethorphan (Coricidin Cough and Cold, Robitussin DM, Drixoral), caffeine pills, Sudafed, diet pills, vitamin supplements, herbal remedies.
How it’s taken: Cough medicines containing DXM are taken orally as a liquid or in capsules, as much as 10-30 times the recommended dose.
Occasional use: You may feel euphoric or disconnected. You may experience heart palpitations, dizziness, blackouts, insomnia, delusions, or seizures, and even coma or death from respiratory distress or heat stroke.
Sustained use over time: Commonly causes nausea, stomach cramps, or other unpleasant gastrointestinal effects that may persist for days after use. At risk for liver, kidney, lung, pancreas, and/or brain damage. Also can cause users to pee blood. Addiction is possible.
Most dangerous when used with: Cold medicines that contain Acetaminophen (like Tylenol), Chlorpheniramine Maleate, MAO Inhibitors and other antidepressants, unless supervised by a physician.

Includes Selective Serotonin Reuptake Inhibitors (SSRIs), MAO Inhibitors, Tricyclics

Such as: Prozac, Zoloft, Celexa, Paxil, Wellbutrin.
How it’s taken: Orally, with the substance building up in the body over time, so one dose does not have immediate effects.
Occasional use: You may not feel much. Most antidepressants take days or weeks to kick in.
Sustained use over time: If you happen to take (unprescribed) antidepressants for long enough to feel the effects, you are in jeopardy of unbalancing the chemicals in your brain and causing emotional disturbances and risk of withdrawal symptoms. The bigger risk actually is for the person whose medication you are taking, who is being deprived of the medication they may need to function optimally.
Most dangerous when used with: Alcohol. Use of any drug will detract from the efficacy of most antidepressants.

Sure, prescription drugs may seem harmless — but misuse of any drug can lead to dependence. Drugs like Ritalin and Concerta, when used incorrectly, can have negative effects. Addiction, paranoia, anxiety, irregular heartbeat, headaches, and even hallucinations are possible side effects, just to name a few.