Final answer:
According to the DEA regulations, a controlled substance log must be retained for at least two years beyond the last entry. This ensures accountability and regulatory compliance for handling controlled substances.
Step-by-step explanation:
The controlled substance log should be kept for a minimum of two years beyond the date of the last entry according to the Drug Enforcement Administration (DEA) regulations. This is a legal requirement aimed at ensuring that records of controlled substances are maintained for an adequate period to prevent misuse and to facilitate accountability. It applies to all registrants who dispense or otherwise handle controlled substances. Healthcare facilities, pharmacies, and other entities that deal with controlled drugs must comply with this rule to aid in regulatory oversight and to ensure legal compliance.