Under what condition may controlled substances be mailed?

Mailing controlled substances is subject to strict regulations established by the Drug Enforcement Administration (DEA) and other authorities to ensure safety and compliance with the law. The correct answer emphasizes the importance of the mailer or addressee being a DEA registrant.

Controlled substances can be mailed only when the individuals or entities involved hold the appropriate registrations. This means that both the sender and recipient must be compliant with federal law, ensuring that they are qualified to handle controlled substances. This regulation protects against misuse and supports accountability in the distribution of these potentially dangerous drugs.

In contrast to this correct option, other choices suggest conditions that do not align with the DEA's guidelines. For instance, limiting mailing only to locations with a pharmacy does not fully address the regulatory framework that allows mailing based solely on the registrant status. The stipulation regarding package size or the requirement for both sender and recipient to be registered physicians also does not reflect the overarching DEA rules governing all registered entities and individuals involved in the distribution of controlled substances. Thus, the clear focus on the necessity of DEA registration for either party ensures compliance with federal laws and promotes responsible handling of controlled substances.