For decades, researchers have ranked the most addictive drugs in the world. A variety of factors influence the addictiveness of a substance, such as the degree of tolerance a person develops after repeated use and the intensity of the drug’s withdrawal symptoms.

The most addictive drugs can cause widespread addiction and public health problems. A number of researchers have conducted studies to rank the addictiveness of specific drugs using a variety of factors.

1. Heroin

Nutt and colleagues ranked heroin as the most addictive drug. Made from extracts collected from the opium poppy plant, heroin is an opioid that is sold as a white or brownish powder. Heroin users commonly inject the drug into their veins to produce a high. It can also be swallowed or smoked.

After the euphoric effects of heroin subside, people often experience a dreamy state during which they are unaware of their surroundings. Heroin use is associated with drowsiness, nausea, respiratory depression, coma and overdose.

Heroin has exacerbated the U.S. opioid epidemic. According to the National Institute on Drug Abuse, more than 15,400 Americans died from a heroin overdose in 2016. The drug is banned in the United States.

2. Cocaine

Cocaine was the next most addictive drug in the study. The stimulant is available on the street as a white crystal powder that can be snorted, injected, smoked or rubbed into the gums. Some people inject a mixture of cocaine and heroin, known as speedball.

Individuals experience an intense rush of euphoria and energy after using cocaine. The drug triggers a buildup of dopamine in the brain, which causes the high. But repeated use can increase a person’s tolerance to cocaine, making them require more of the drug to achieve the desired effects.

Some surgeons prescribe cocaine for medical purposes, but recreational cocaine use is illegal in the United States. The drug can lead to addiction, overdose or death. In 2016, more than 10,600 Americans died of cocaine overdose.

3. Nicotine

Tobacco use includes smoking, chewing or sniffing products that contain nicotine. According to the U.S. Department of Health and Human Services, about 20 percent of adults and teenagers smoke.Nicotine can be found in a host of tobacco products, including:

  • Cigarettes
  • Cigars
  • Hookah products
  • Bidis
  • Kreteks

Tobacco use harms your physical health. More than 16 million people have contracted at least one disease because of smoking. Since 1964, more than 20 million people in the United States have died because of smoking.

4. Street Methadone

Nutt’s study ranked street methadone as the fourth most addictive substance. In clinical settings, addiction experts provide methadone to wean people off opioids. The drug is designed to reduce cravings and prevent opioid withdrawal symptoms.

When taken as prescribed, methadone is generally safe and effective. Many people receiving methadone treatment can continue to contribute to society. But when the medication is misused, it can lead to lightheadedness, hallucinations and addiction.

Street methadone is methadone sold to someone without a prescription. The drug does not produce a high unless it is taken in significantly high doses, but it can lead to overdose. Many street methadone users take the drug to alleviate opioid withdrawal symptoms.

5. Barbiturates

Barbiturates depress the central nervous system, causing euphoric and sedative effects. These drugs can reduce anxiety and inhibitions, but they can also impair memory and increase irritability and paranoia.

People who use barbiturates can easily develop a tolerance. Misusing this class of drugs can lead to severe health problems, including addiction and overdose. Signs of barbiturate overdose include clammy skin, dilated pupils, shallow breathing and coma.