This lecture is intended as a 'no holds barred' introduction for the interested layperson, be they casual observer, user or worried parent. During it, we will explore how a wide variety of drugs work on the brain and nervous system altering behaviour. Following an introduction to nerve cells, synapses, neurotransmitters and brain anatomy, we will turn our attention to how drugs change the way in which nerve cells communicate. We will describe not only the neural and cerebral effects of recreational drugs such as caffeine, alcohol, ecstasy and heroin, but also the effects of prescription compounds such as antidepressants and antipsychotics. Finally, we will consider whether or not cannabis is as harmful to young people as is often stated, presenting a reasoned review of recent scientific studies examining its effects on the brain and its association with mental illness.