Final answer:
When an oral medication's specific action isn't known, gastrointestinal side effects are a common consideration due to the digestive processing of orally administered drugs.
Step-by-step explanation:
In medicine, the term "PO" stands for per os, which means the medication is taken orally. When discussing potential side effects of a medication, it is important to consider the route of administration. If a medication is taken orally, it is absorbed through the gastrointestinal tract, which can lead to gastrointestinal side effects.
Therefore, if you don't know what a medication does and it is taken PO, the most likely side effect would be gastrointestinal in nature. For example, drugs taken orally can cause nausea, vomiting, or stomach upset.