"• In around 11 per cent of drug poisoning deaths only a general description is recorded on the coroner’s death certificate (such as drug overdose or multiple drug toxicity). Deaths where the certificate contains only non-specific information cannot contribute to the counts of deaths involving specific substances.
"• In an additional 34 per cent of all drug poisoning deaths, the death certificate mentions more than one specific drug. Where more than one drug is mentioned, it is not possible to tell which was primarily responsible for the death.
"• Where more than one drug is mentioned on a death certificate the death will be counted in more than one category in Table 3. For example, if both heroin and cannabis are mentioned, the death will be recorded once under heroin and once under cannabis. Therefore the numbers for different substances cannot be added together to give a total number of deaths.
"• Approximately 30 per cent of all drug-related poisoning deaths also contain a mention of alcohol or long-term alcohol abuse (for example, cirrhosis) in addition to a drug."


"Statistical Bulletin: Deaths Related to Drug Poisoning in England and Wales, 2011" (London, England: Office for National Statistics, Aug 29, 2012), p. 12.