What term is commonly used for a large dose of medication taken all at once?

The term "overdose" specifically refers to administering a dose of medication that exceeds the recommended or prescribed amount. In the context of the question, when a large dose of medication is taken all at once, it typically implies that the amount ingested is more than what is considered safe or appropriate for an individual. This situation can lead to toxic effects and serious health complications.

The other terms provided have different meanings in the medical context. A "regimen" refers to a systematic plan for administering a treatment or medication over a specified period, often detailing the frequency and amount of doses. "Protocol" signifies a formal set of guidelines or procedures to be followed in clinical practice. "Administration" refers to the act of giving or applying medication or treatments, but does not imply the amount taken or whether it is considered safe. Therefore, "overdose" is the most appropriate choice to describe a large and potentially harmful dose of medication taken at one time.