- Some recent studies linking vaping to heart disease and cancer rely on weak or inconsistent methods.
- Experts say “adjusting for smoking” is often done too crudely to separate vaping risks from decades of past smoking.
- Reviews that combine flawed studies can give a false sense of scientific certainty.
- Poorly designed studies can mislead the public by treating vaping as if it carries the same risks as long-term smoking.
Headlines about vaping and serious disease can be frightening. Heart attacks. Strokes. Cancer. When studies appear in respected journals making these links, it’s easy to assume the science is settled.
But many experts say it isn’t – and that some widely cited studies should be treated with caution, not because vaping is risk-free, but because the research methods behind the claims are shaky.
Two recent cases help explain why.
The smoking history problem
One high-profile review claims that people who use vapes have a higher risk of heart attacks and strokes. To address the obvious issue – that most people who vape used to smoke – the authors say they “adjusted for smoking.”
That sounds reassuring. In reality, critics say it often means little more than sorting people into broad boxes: never smoked, former smoker, or current smoker.
Harm reduction expert Clive Bates says that approach should “ring immediate alarm bells.”
“How would we even know the answer to the question?” he asked. “It is not simply a matter of ‘adjusting for smoking’ as if that is a practical thing to do.”
Heart disease doesn’t work in neat categories. Risk builds up slowly over decades. Someone who smoked for 30 or 40 years, then spent a short time dual-using cigarettes and vapes before switching entirely, carries that long-term damage with them.
“The effects of these exposures are cumulative and progressive,” Bates said. “How would you adjust for 35 years of smoking, followed by three years of dual use, then two years of exclusive vaping? That total history is the actual exposure.”
Labeling someone simply as a “former smoker” doesn’t erase decades of harm. But many studies don’t even record basic details such as years smoked, when someone quit, or how long they’ve been vaping.
Bates said: “Generally, the ‘adjustment’ is just for never, former, or current smoker status. That doesn’t capture cumulative lifetime exposure.”
As a result, harms caused by years of smoking can end up being blamed on vaping, especially when vaping is relatively recent. Notably, in these studies, the strongest signals of harm tend to show up among former smokers, not among people who never smoked at all. To many experts, that’s a red flag.
As one reviewer noted, the only significant associations appeared among former smokers, not among never-smokers. This suggests the findings may reflect lingering effects of past smoking or other confounding factors rather than vaping itself.
Cause and effect can get flipped
There’s another issue that’s easy to miss. Many of the studies used in these reviews look at people at a single point in time. That makes it hard to know what came first.
Did vaping lead to poor health? Or did people who already felt unwell, after years of smoking, switch to vaping in hopes of reducing harm?
“Reverse causation can never be ruled out,” Bates said. “People beginning to feel ill as a result of smoking may take up vaping to mitigate their risk. How is that handled?”
In short, it often isn’t. This problem can’t be solved with statistics alone. You need long-term studies that follow people over many years, tracking smoking histories, switching patterns, and health outcomes over time. Those studies don’t yet exist for vaping.
When reviews magnify weak evidence
Systematic reviews and meta-analyses are often described as the strongest kind of evidence. But they’re only as good as the studies they include.
If most of those studies share the same weaknesses – poor measurement of smoking history, short follow-up, unclear timing – then combining them doesn’t fix the problem. It can actually make the results look more definitive than they really are.
“Systematic review and meta-analysis is often code for ignoring the flaws that are common to all the studies included,” Bates warned.
A cancer review under fire
Concerns become even stronger in the case of a recent review linking vaping to cancer, which prompted a detailed letter to the editor from independent researchers.
The authors say they identified “undisclosed protocol deviations, inconsistencies between the reported search strategy and the included evidence, misclassification of study designs, internal contradictions, numerical discrepancies, and unsupported conclusions.”
Among the problems they cite are changing the study rules midstream without disclosure, adding study types that were originally excluded, and mixing cancer incidence with biomarkers that do not measure cancer at all.
“Admitting biomarker or inflammatory marker studies under the category of cancer incidence creates a disconnect between the stated objective and the evidence included,” the letter says.
Even more troubling – a study that had already been retracted appeared in the review, and was still rated as high quality. Basic numbers didn’t add up either, including cancer case counts that exceeded the total number of participants.
“These are not minor oversights,” the authors wrote. “They undermine the transparency, accuracy, and interpretability of the review.” Taken together, they argue, the evidence base is far too inconsistent to support claims that vaping increases cancer risk.
What all this means
None of this means vaping is completely harmless. Nicotine is addictive, and inhaling chemicals isn’t risk-free. But it does mean that claims about vaping causing heart attacks or cancer should be based on solid, transparent science, especially when they’re used to shape public policy or alarm consumers.
