Final answer:
The DEA mandates that a new pharmacy must take a complete inventory of drugs initially and then every two years, which is also known as biennially.
Step-by-step explanation:
The Drug Enforcement Administration (DEA) requires that a new pharmacy takes a complete inventory of drugs initially when it begins operations and thereafter biennially (every two years). This means that if a pharmacy opened in January 2021, they would need to take their first inventory before they officially begin dispensing controlled substances and then they would need to conduct their next inventory in January 2023, and every two years following. It is crucial for new pharmacies to comply with this rule to ensure they are within legal regulations and able to accurately track controlled substances.