Drug addiction is a growing problem across the globe, but what is the most addictive in the range of illicit drugs available today? It basically varies, since some researchers rely on the potency of the drug and the harm it causes.

In 2007, drug expert David Nutt from the UK and his co-researchers asked a panel of addiction experts to score each drugs by how addictive it was. Dr. Eric Bowman, a psychology and neuroscience lecturer, ranked the top five most addictive drugs on Earth.

1. Heroin

Heroin is one of the most popular addictive substances on Earth. It scored 2.5 out of a maximum addictive score of 3. It is a type of opioid drug that is synthesized from morphine and can increase the reward system of the brain by up to 200 percent in laboratory animals.

In 2011, 4.2 million Americans older than 12 years old had used heroin at least once in their lives. About 23 percent of those who use heroin become dependent on it.

It's a dangerous drug because its lethal dose is only five times greater than those required for a high. There are many celebrities who were believed took in lethal doses of heroin that led to their untimely death.

2. Alcohol

This is the only legal addictive substance in the United States and United Kingdom. It is ranked second most addictive substance with a score of 2.2 out of 3. Alcohol abuse can damage the brain and other major vital organs like the heart, pancreas and liver.

For pregnant women, it predisposes the infant to various developmental problems. Alcohol consumption also increases the risk of cancers.

Experiments on laboratory animals show that it can increase dopamine levels in the brain's reward system by 40 to 360 percent. The more these animals were given alcohol, the more dopamine levels increased.

According to the World Health Organization (WHO), the harmful use of alcohol led in 3.3 million deaths annually. The worldwide total consumption of alcohol is equivalent to 6.2 liters (1.6 gallons) of pure alcohol per person among 15 years and older in 2010.

3. Cocaine

Cocaine is a short-acting stimulant, which can lead users to "binge," meaning taking the drug as many times in a single session. This substance interferes with the brain's use of dopamine to convey messages from one brain cell to another. This prevents neurons from switching off dopamine signals, leading to an abnormal activation of the brain's reward pathways.

This further leads to severe medical consequences associated with organs of the body such as the heart, lungs, brain and digestive system. Users can also experience severe paranoia and can lose touch with reality.

In laboratory animals, cocaine use resulted in increased levels of more than three times the normal level of dopamine.

4. Barbiturates or "Downers"

Downers or barbiturates shut down the brain's functions to make people feel happier. These substances are very dangerous because high doses can be fatal since they also suppress breathing.

This substance interferes with the chemical signals in the brain. At low doses, it may cause happiness or feeling of euphoria. Though the study listed this drug as 4th in the list, Bowman suggests that these drugs are less widely-used at present.

The dependence on this substance declined since prescriptions are now difficult to acquire.

5. Nicotine

Just like alcohol, nicotine is also one of the most commonly-used chemicals since it can be found in cigarettes and tobacco.

Nicotine is rapidly absorbed in the lungs and transferred to the brain, which increases the risk of cancer, respiratory diseases and cardiovascular problems. More than two-thirds of Americans who tried smoking reported that they became dependent during their life.

WHO estimates that were more than 1 billion smokers in 2002 and the number continues to grow up to date.

The study was published in the journal The Lancet.

Photo: Kaushik Narasimhan | Flickr