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 will we go after this life? Whether we were created by an omnipotent being as in many religions, we just happened to be by chance and got to where we are through evolution, or maybe even something else. Whatever the case, humans have always been curious and wondered about our existence and consciousness. Along with these age-old questions, 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 the functioning of the brain, causing changes in behavior, mood and consciousness. There are different types of psychoactive drugs; some of these include: stimulants, depressants, and most importantly, hallucinogens or psychedelics. Humans have been using these different types of drugs since before recorded history. These drugs are used for a variety of reasons such as 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. The use of illicit drugs is more frequent among males and young people (WHO). The brain is protected by a thick covering of cells called the blood-brain barrier. However, some drugs are able to pass through and affect 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 acro space...... in the center of the paper......, and the classic hallucinogens: Identification of a new therapeutic class: the Entactogens.”Journal of Psychoactive Drugs, Vol 18(4), October-December 1986, 305-313.NIDA. “NIDA for Teens: Drug Facts – Stimulants.” NIDA for Adolescents: The Science Behind Drug Abuse. NIDA, 2011. Web. December 10, 2011. “Psychedelic Uses.” Psychedelic adventure. 2011. Network. 10 December 2011. .Trulson ME, Ross CA, Jacobs BL. “Behavioral evidence for stimulation of serotonin receptors in the central nervous system by high doses of LSD.” Psychopharmacology communications. 1976;2(2):149-164.WHO. "Scientific facts about psychoactive drugs, tobacco, alcohol and illicit substances". GreenFacts - Facts about health and the environment. Green Facts, 2006. Web. 10 December. 2011. .