When you add too much yeast during the brewing process, chances are good that your beer will be fine. It is difficult to overpitch beer in a way that negatively affects the final product.
In reality, it is difficult to overpitch when homebrewing. Even when you do add more yeast than necessary, it is not considered too much as it may change your beer but shouldn’t ruin it.
A good rule of thumb for homebrewing is to pitch a little more than you think you’ll need. Underpitching will have worse effects and is much easier to do.
Overpitching can result in faster fermentation which can, in turn, reduce ester production. It can also lead to a dryer beer.
Any beer style that gets most of its flavor from yeast should not be overpitched, including Hefeweizens and Belgian-style Tripels. You should also avoid overpitching ale yeasts such as WLP400.
Adding more yeast does not add more alcohol. Yeast consumes sugar to produce alcohol and carbon dioxide. The alcohol content is limited by the amount of sugar in your wort.
The two best sugars for homebrewing are dextrose and sucrose. These two sugars are 100% fermentable and relatively simple for yeast to consume.
Additional yeast can speed up fermentation because fermentation lasts as long as there are available nutrients. More yeast will consume the nutrients quicker.
Luckily, adding yeast is not a factor that can result in toxic beer. In fact, a good yeast pitch can reduce the chances by speeding up fermentation.