You cannot legally add arsenic or strychnine to beer.

The Law

During the early days of mass brewing, unscrupulous brewers would sometimes use toxic chemicals like arsenic to alter the flavor or clarity of their beer, or strychnine to give it a bitter bite without using hops. This shocking practice led to massive public health scares and fatalities. The state was forced to write a law explicitly pointing out that poisoning your customers' beer was illegal.