Unlike the pure opioids such as morphine and heroin, opium is a complex and variable mixture of substances reflecting differences in both the starting material and the traditional practices of the regions in which it is produced. Analytical methods have improved greatly in recent years, to the point that the source of a preparation can often be identified by its opioid content and its impurities. Daily amounts used, both by smoking and by mouth, vary widely from less than a gram to 30 g, equivalent to 75-3000 mg of morphine. The effects of opium are essentially those of morphine but unexpected toxicities, such as oesophageal cancer associated with "dross opium" and polyneuropathy due to deliberate addition of arsenic, are problems in some specific regions. Prevalence of use in different areas and countries is governed by the same factors of ease of availability, price and social acceptance that apply to the use of alcohol and other drugs in western countries. The risk of addiction to opium smoking appears to be somewhat less than to parenteral use of heroin, but appreciably greater than to alcohol. Even in countries where its use is traditional, opium smoking carries substantial risks of harm to health and social functioning.