If you look at the CDC's mortality data, you will see that most records contains several causes of death. The death certificate is filled out free-form, in the sense that the people filling them out aren't forced to pick from a drop-down menu of lists of causes. The medical examiner can write whatever they want on it, so long as it's reasonably comprehensible and in plain English and generally complies with the structure of a death certificate (I won't get into details here). When the CDC gets these death certificates, it encodes the free-form death certificates with pre-specified codings. So "OD'd on oxycontin", "empty bottle of hydrocodone", and "toxicology found extremely high levels of oxymorphone" would all be coded as T40.2, the ICD-10 code for "other opioids." (I haven't seen a sample of actual death certificates, so I don't know what they typically look like when filled out long-form. But I've spent a lot of time examining the coded file.)
Something that was apparent right away was that many of these deaths have contributing factors beyond the drugs themselves, possibly alternative causes of death. The CDC file has 20 columns for specifying contributing causes of death. (Most only use, say, three to five columns, and I've seen up to 16 used. I'm not sure the full 20 are ever used.) I created a list of all the overdose deaths from the CDC's mortality files (details here; I do a write-up every year when these come out). Then I stacked all 20 of the contributing cause of death columns into one column, and I counted the frequency of each cause. I end up with a list of causes of death that looks like this:
Nothing too surprising. You see all the drugs and drug categories for which ICD-10 has a code. The drugs most commonly involved in drug poisonings are at the top of the list (synthetic narcotics has 33,985 mentions in 2017, Heroin has 18,276 mentions, etc.). And various codings for persistent drug problems, whose descriptions all start with "Mental and behavioral disorders due to...", are highly represented. But then you start seeing things like "Atherosclerotic cardiovascular disease, so described." Why is this on here? Why is it so common? Is it showing up because medical examiners are writing "overdose was exacerbated by ..." and then listing such a medical condition? Or are the death certificates clearly specifying that this was a drug overdose, then gratuitously adding details about the health status of the decedent? There is supposed to be a structure and a logical flow to a death certificate. They aren't generally supposed to add gratuitous information, but rather they are supposed to specify a causal chain. (If there is a causal chain, like cocaine intoxication -> car accident or opioid overdose -> anoxic brain damage, then these details are supposed to be filled out on the various lines of a death certificate in a certain standard order.)
I took the full list and attempted to whittle it down to just the chronic illnesses. I deleted the things that are obviously drug categories (Heroin, Cocaine, etc.) and the "Mental and behavioral disorders..." listings. I also tried to eliminate anything that looked like the physical effects of a drug overdose. For example, "anoxic brain damage, not elsewhere classified" shows up quite a lot. The mechanism by which opioids kill people is suppression of respiration. The body breaths to slowly to get enough oxygen. "Anoxic brain damage" on, say, a heroin overdose is just specifying in gory detail how a drug overdose kills someone. So I deleted terms like this from my list (also, "asphyxiation", "respiratory failure, unspecified" and "asphyxia"). My intention is to get a list of chronic illnesses or other alternative causes of death that were listed on the certificates.
There are a couple of things going on here. Some of these conditions are obviously risk factors in an opioid overdose. People with obesity and sleep apnea have trouble breathing properly, so they should be especially susceptible to a drug that suppresses respiration. I'm not as sure about "cardiomegaly", or "atherosclerotic heart disease." For whatever reason, the medical examiners thought these conditions were relevant enough to list on the death certificate. I don't know if they're saying this condition exacerbated the effects of an opioid overdose, or if maybe these are alternative causes explaining the death, or if they're somehow involved in the chain of events leading up to the person's death, or if the examiner is just gratuitously listing information not relevant to establishing cause of death. What is clear is that many of these people were already sick, and moreover sick enough that their illness warranted mention. And that makes me question how many of these were actually "opioid poisoning" deaths versus deaths from chronic illness in which the patient happened to be using opioids.
It does not seem to be the case that medical examiners gratuitously mention details about drugs on the death certificate unless they think it's a drug poisoning. Take a look at my earlier post, which is mainly about drug poisonings by intent (suicide vs. accidental). The "other" category includes non-poisoning deaths. These make up about 6% to 12% of drug-involved deaths, depending on the year. Inspecting some of these, you see a lot of deaths where drugs were obviously some kind of factor. There are a lot of automobile accidents and bath-tub drownings, for instance. But there aren't a whole lot of death certificates that mention drugs when they are just incidental. A mention of drugs almost always implies that they are causally connected to the death. Given that, I'm scratching my head at all these "incidental" mentions of chronic diseases on drug poisoning deaths. Is there a kind of reporting bias in which any finding of drugs means the death ends up getting labeled a drug overdose? Or do medical examiners scrupulously avoid mentioning incidental findings of drugs when they think there is some other cause present? (If "yes" to that last one, why do they list incidental information about chronic illnesses so frequently?)
It would be an interesting project for someone with access to the CDC's internal systems to take a closer look at how these chronic illnesses are ending up on the death certificates. It's possible that a representative sample of the actual death certificates would paint a clearer picture. I have to settle for the data after it's been converted from raw text and coded under ICD-10. I think there is a very rich vein of research here that would illuminate a not-often-discussed driver of the opioid crisis, or possibly uncover a reporting bias that's inflating the official numbers.
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Here was my list of chronic illnesses and other alternative causes of death: Hypertensive heart disease without (congestive) heart failure ; Atherosclerotic cardiovascular disease, so described ; Cardiomegaly ; Atherosclerotic heart disease ; Essential (primary) hypertension ; Obesity, unspecified ; Chronic obstructive pulmonary disease, unspecified ; Cardiac arrest, unspecified ; Unspecified diabetes mellitus, without complications ; Drowning and nonfatal submersion ; Cardiac arrhythmia, unspecified ; Other and unspecified cirrhosis of liver ; Other obesity ; Pneumonia, unspecified ; Emphysema, unspecified ; Congestive heart failure ; Acute myocardial infarction, unspecified ; Asthma, unspecified ; Chronic viral hepatitis C ; Bronchopneumonia, unspecified ; Dilated cardiomyopathy ; Septicaemia, unspecified ; Fatty (change of) liver, not elsewhere classified ; Other specified cerebrovascular diseases ; Pulmonary oedema ; Other chronic pain ; Injury, unspecified ; Toxic effect of carbon monoxide ; Anxiety disorder, unspecified ; Insulin and oral hypoglycaemic [antidiabetic] drugs ; Bipolar affective disorder, unspecified ; Pulmonary embolism without mention of acute cor pulmonale ; Unspecified multiple injuries ; Intracerebral haemorrhage, unspecified ; Cardiomyopathy, unspecified ; Heart disease, unspecified ; Atrial fibrillation and flutter ; Hypothermia ; Exposure to excessive natural cold (hypothermia) ; Sleep apnoea ; Chronic ischaemic heart disease, unspecified ; Non-insulin-dependent diabetes mellitus, without complications ; Exposure to excessive natural heat (hyperthermia) ; Gastrointestinal haemorrhage, unspecified ; Hyperlipidaemia, unspecified ; Acute renal failure, unspecified ; Other specified symptoms and signs involving the circulatory and respiratory systems ; Chronic renal failure, unspecified ; Hepatic failure, unspecified ; Effect of heat and light, unspecified ; Stroke, not specified as haemorrhage or infarction ; Cardiovascular disease, unspecified ; Unspecified renal failure ; Heart failure, unspecified ; Chronic kidney disease, stage 5 ; Open wound of head, part unspecified ; Unspecified diabetes mellitus, with ketoacidosis ; Endocarditis, valve unspecified ; Other hypertrophic cardiomyopathy ; Effect of reduced temperature, unspecified ; Obstetric death of unspecified cause ; Dorsalgia, unspecified ; Dissection of aorta [any part].
I sorted the list from most common to least common and went down the list, deleting obvious drug-related causes and such. I stopped when I got 100 or so, because the full list is in the thousands. I would have to go through by hand and manually curate the list to get a more accurate number. So the 31.8% given at the top of this post is a bit low; it's not counting the full list of chronic illnesses found on these death certificates. But it's counting the most common ones, so it's not an egregious under-count.
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