How does smoking affect testosterone?
The short answer is that we don’t know for sure.
The slightly longer answer?
Separate scientific studies of smoking and testosterone provide different results — some show no effect, some show higher testosterone levels in smokers, and others show the opposite. A meta-analysis of data from lots of these separate studies found that testosterone levels are higher in men who smoke than in non-smokers.
The more complete answer
Most of the studies of smoking and testosterone compare levels of the hormone between groups of men who smoke with levels in a group or groups who either never smoked and/or are ex-smokers.
Studies like these can’t account for possible differences in why people smoke in the first place, and how those factors might affect testosterone levels. For example, some studies suggest that people whose mothers smoked during pregnancy are more likely to smoke themselves and that maternal smoking might increase testosterone levels in sons.
Ideally, to find out if smoking affects testosterone levels, you’d measure testosterone in men who don’t smoke, then randomly start half of them smoking and measure their testosterone levels after they’ve been smoking (or not) for a while. That’s obviously unethical, given the known health dangers of smoking.
Another factor that limits the value of existing studies of smoking and testosterone is the method used to measure levels of the hormone in blood samples. None of the studies included in the meta-analysis mentioned above used the gold standard method for measuring testosterone.
The limitations of the available evidence do not allow us to conclude that smoking causes an increase in testosterone.
However, there are theoretical ways in which smoking might increase testosterone. For example, the breakdown of nicotine and its metabolites might slow down the breakdown of testosterone in the body, which could have the effect of increasing testosterone levels in the blood. There’s no scientific data to support these theories, though.
Even if smoking does increase testosterone levels, whether or not this would be beneficial or harmful is unknown. Of course, on balance, smoking is one of the worst things you can do for your health.