Scientists aren’t quite sure why humans have a biological affinity for certain drugs. Why, for example, is there a receptor in the central nervous system that so readily accepts opioids and other drugs? The fact is that these substances can and do become drugs of abuse for millions of people around the world. Here are some of them:

Alcohol

Alcohol is the most popular mind-altering drug and the second most popular mood-altering drug. Only caffeine is ingested more widely. It is different from the other drugs listed here because it is socially acceptable in many parts of the world. It is a social lubricant that has health benefits if drunk in moderation. However, the consequences of severe alcohol abuse can be dire. They include damage to the liver and brain and behavioral changes that can destroy a person’s work and family life.

Alcohol passes through the blood-brain barrier and causes the feeling of intoxication by interacting with neurotransmitters. As such, alcohol is a central nervous system depressant.

Opioids

Opioids are both natural and synthetic and have been used for centuries to ease the pain. Like alcohol, opioids are central nervous system depressants. The “standard” opioid is morphine, which is a natural opioid derived from the opium poppy. It is quickly absorbed through the mucous membranes and so is effective whether it’s snorted, injected or smoked.

Opioids include heroin, codeine, naloxone, oxycodone, fentanyl and its derivative, carfentanil. Carfentanil is 10,000 times more potent than morphine and has contributed to the 33,000 opioid deaths that occurred in 2015 in the United States.

Heroin

Named for the “heroic” way it made patients feel, heroin was developed in 1874 as an alternative to morphine. However, it is a much more potent drug though not nearly as powerful as more conventional opioids. It is a fact acting and crosses the blood-brain barrier more quickly than morphine does. Like morphine, heroin can be smoked, snorted or injected, though it is usually injected after being diluted with water. Street heroin is often adulterated, or “cut” with carfentanil.

Methamphetamine

Unlike opioids, methamphetamine is a central nervous system stimulant. Its use is almost entirely recreational, though some forms are used a nasal decongestant. It causes the user to feel great rushes of energy and enormous elevations in mood and libido. However, constant abuse of methamphetamine can lead to psychosis, convulsions and brain bleeds. Meth addiction can damage the brain by destroying areas of gray matter.

Cocaine

Cocaine is another powerful stimulant. It produces feelings of great happiness and energy, but these states are often accompanied by paranoia, rapid heartbeat, elevated body temperature and an increased risk of heart attack, stroke and sudden death. Cocaine can be snorted, injected or smoked, and the effects usually begin within minutes or seconds after it’s ingested. They last from half an hour to an hour. Cocaine is the second most popular illegal, recreational drug. Only marijuana is used by more people.

Ecstasy

Ecstasy or MDMA is famous for its ability to help users feel great empathy, inner peace, and euphoria. It can also cause mild hallucinations. These effects begin 30 to 45 minutes after the drug is ingested and last for as long as six hours. Ecstasy is a purely recreational drug. As of 2017, there is no medicinal use for it, though it was once used as an aid in psychotherapy. Death from ecstasy comes because of dehydration or substantially elevated body temperature.

Hallucinogens

Though hallucinogens are associated with the 1960s, they have been used throughout history. There are natural drugs such as psilocybin mushrooms, peyote cactus, and even jimsonweed. Most people think of synthetic drugs such as LSD when they think of hallucinogens. These drugs are different from the others discussed because of the extreme effects they have on the user’s perception. How a hallucinogen is experienced does not depend so much on the dosage but the setting and the person taking it.

Inhalants

These drugs are volatiles that alters the mind when they are inhaled abusively. They are unpredictable, which makes them particularly dangerous. They can include anything from aerosol propellants to gasoline, laughing gas and even the nitrogen found in cartridges for whipped cream dispensers.

Marijuana

Only alcohol is a more widely used drug than marijuana. It is famously smoked, and millions of Americans start using the drug every year. In some areas, it has been decriminalized. The short term effects of marijuana depend on the amount of its active ingredient, delta-9-tetrahydrocannabinol, or THC. Effects are an altered state of consciousness, a gentle type of euphoria, clumsiness, increased appetite and increased heart rate.

Club Drugs

These are drugs taken most by young people who wish to enhance their experience of a night on the town. They include methamphetamine, ecstasy, ketamine, Rohypnol, GHB, and LSD. The effects of these drugs vary considerably.

Anabolic Steroids

These drugs are testosterone-like hormones that support the growth of muscles. They are used by athletes, particularly bodybuilders. When abused, anabolic steroids can lead to liver disease, stroke, wild mood changes, heart attacks and elevated risk of suicide.

Prescription Drugs/Cold Medicines

Prescription drugs that are abused are often types of opioids. Oxycodone, a narcotic prescribed to treat the type of pain experienced after surgery or by cancer patients, is a notoriously abused prescribed drug. A cough and cold medicines often contain codeine, another opioid. Stimulants such as Ritalin can also be misused.

Synthetic Cannabinoids

These drugs are named because they are related to the chemicals found in marijuana. They are sprayed on plants to be smoked or inhaled as incense or in e-cigarettes.

Bath Salts

These drugs are related to cathinone, an ingredient in the khat leaf that produces a mild euphoria when chewed. They are considered inexpensive substitutes for methamphetamine, ecstasy, and cocaine.

Tobacco/Nicotine

Like other natural substances used as drugs, tobacco’s history is long and complicated, with humans smoking the plant as long ago as 6000 BC. Native Americans believed the plant was health giving and could cure a toothache, burns, and tumors. Europeans and Chinese thought it could cure the plague. Now people are aware that tobacco is the primary source of nicotine, a highly addictive drug, and tobacco smokers are at much greater risk of dying from a variety of cancers, especially lung cancer.