The twentieth century saw a remarkable upsurge of research on drugs, with major advances in the treatment of bacterial and viral infections, heart disease, stomach ulcers, cancer and mental illnesses. These, along with the introduction of the oral contraceptive, have altered all of our lives. There has also been an increase in the recreational use and abuse of drugs in the Western world. This Very Short Introduction, in its second edition, gives a non-technical account of how drugs work in the body. Reviewing both legal (alcohol, nicotine and caffeine) and illegal drugs, Les Iversen discusses why some are addictive and whether drug laws need reform.