Why the combination is special
For cancers of the mouth, throat, voice box and oesophagus, alcohol acts partly as a solvent. It helps tobacco carcinogens dissolve and penetrate the cells lining your mouth and throat. So the two exposures do not just stack, they amplify each other.
The number that surprises people
Heavy smoking combined with heavy drinking pushes head and neck cancer risk to roughly 35 times that of someone who does neither, according to the INHANCE consortium and a 2024 meta-analysis. If the two risks simply added together, you would expect something closer to 15 times. The gap between 15 and 35 is the synergy.
The US National Cancer Institute describes the combined effect at these sites as multiplicative, and INHANCE measured a joint effect that was actually greater than multiplicative.
The useful part
Because the effect is multiplicative, cutting either one down breaks the multiplier. You do not have to quit both to meaningfully drop the combined risk. Reducing one side pulls the whole product down.