Throughout the history of the human species, we have always wanted to know three fundamental questions: why are we here, where do we come from and where are we going after this life? Whether we were created from an all-powerful being as in many religions, we happened by chance and got to where we are through evolution, or maybe even something else. Regardless, humans have always been curious and questioned about our existence and consciousness. Along with these age-old issues, also came the use of psychoactive drugs and other mind-altering substances. To try to understand these questions, humans have experimented with drugs that alter their state of consciousness. Psychoactive drugs are defined as chemicals that affect brain function, causing changes in behavior, mood, and consciousness. There are several types of psychoactive drugs, including: stimulants, depressants and, most importantly, hallucinogens or psychedelics. Humans have used these different types of drugs since before recorded history. These drugs are used for a variety of reasons, including medical purposes, religious practices, spiritual growth, and even recreation. Worldwide, approximately 200 million people use some type of illicit drug, most commonly cannabis or better known as marijuana. Illicit drug use is more common among men and young people (WHO). The brain is protected by a dense layer of cells called the blood-brain barrier. However, some drugs are able to pass through and influence the functioning of the brain. These drugs alter consciousness by facilitating or inhibiting synaptic transmission. A synapse is a junction between two nerve cells, consisting of a tiny space in the middle of paper ......, and classic hallucinogens: Identification of a new therapeutic class: Entactogens. "Journal of Psychoactive Drugs, Vol 18(4), October-December 1986, 305-313.NIDA. "NIDA for Teens: Facts about Drugs - Stimulants." NIDA for Teens: The Science Behind Drug Abuse . NIDA, 2011. Web. December 10, 2011. “Psychedelic Uses.” Internet. December 10, 2011. “Behavioral Evidence for CNS Serotonin Receptor Stimulation.” by high doses of LSD”. Psychopharmacological Communications 1976;2(2):149-164. WHO “Scientific facts on psychoactive drugs, tobacco, alcohol and illicit substances.” and the environment. Green Facts, 2006. Web December 10.. 2011. .