A common misconception is the so-called 'cocktail effect' - the idea that when different chemicals combine, their total effect is greater than might be expected.
It's called the cocktail effect because of the belief that mixing alcoholic drinks can be more potent than drinking just one type of drink on its own (actually, it isn't true). The reality is that when we are exposed to a variety of chemicals at the same time, the result can be simply additive, or it may indeed be enhanced, but it might even be diminished with one substance cancelling out the effects of another. Scientists can and do investigate whether substances will have additive effects or synergistic effects or even cancel one another out when devising their formulations. These findings are taken into account when assessing the safety of a product.
Remember that even a simple cup of tea is in fact a cocktail of more than 200 chemicals, plus milk and sugar.