Most practitioners and businesses that handle controlled substances are required to hold a controlled substances registration with the Drug Enforcement Administration. This includes physicians, pharmacies, dentists, hospitals, manufacturers, reverse-distributors, nurse practitioners, and virtually everyone else who possesses, prescribes, manufactures, or dispenses controlled substances.  Depending on the state, the practitioner or entity may also be required to hold a state-level registration issued by the applicable state entity. For example, until 2016 practitioners and entities in Texas were required to hold a state controlled substances registration issued by the Department of Public Safety before they could even apply for a DEA registration.
Any practitioner or business holding a controlled substances registration is required to comply with federal law including the Controlled Substances Act and the applicable rules in the Code of Federal Regulations. While many of these rules are fairly easily understood there are many more that are grey and open ended. Violation of these rules can subject a registration holder to both civil and criminal liability. On the civil side this can involve very large fines and/or the revocation of the holder’s controlled substances registration through a show cause hearing. Criminal penalties can be severe and include incarceration. 
The DEA is charged with enforcement of these laws and often works in concert with the United States Attorney’s Office, state law enforcement, and state-level regulators. The DEA’s self-stated mission is to ensure the closed circle of distribution for controlled substances remains intact. This means making sure controlled substance stay within the lawful possession and control of legitimate users from its initial manufacture to final use by a patient. Diversion occurs when a controlled substance leaves the closed circle of distribution through theft, illicit use by the registration holder or their employee, non-therapeutic prescribing by a practitioner or other event that involves in the drug being diverted from legitimate channels. 
In my experience the DEA is very aggressive in its investigations and enforcement actions. This has only increased in recent years as it responds to the national opioid crisis. This has led to an increased emphasis on policing controlled substances registration holders as opposed to the DEA’s historical focus on illegal drug trafficking. 
Oftentimes a registration holder does not receive notice they are being investigated by the DEA until the local DEA field office suddenly arrives at their business or home accompanied by local law enforcement and state regulators. This is usually done in the context of serving a warrant to seize records, cash, and other property that is suspected to be the proceeds of illegal activity. The DEA will also typically try to conduct on-the-spot interviews with the registration holder and their employees before they have an opportunity to consult with an attorney. This is often accompanied by a demand that the practitioner or entity immediately surrender their controlled substances registration, again without having the benefit of discussing this with a lawyer.
In other cases a practitioner or business entity will first become aware they are the subject of DEA scrutiny during or after a routine DEA audit. If the auditor finds discrepancies or other problems the registration holder will typically be notified in writing of the suspected violations. This can often be accompanied by a civil penalty demand and/or request for surrender of the registration accompanied by a threat the DEA will otherwise pursue revocation of the registration through a show cause hearing. The civil penalty demands can be quite massive as federal law allows each violation to be counted as a separate occurrence worth thousands of dollars in penalties. While these penalties can often be negotiated by an attorney the total can still be financially crippling to the registration holder.
Any practitioner, pharmacy, or other business that has been contacted by the DEA should promptly consult with a defense attorney. If the DEA suddenly shows up at your doorstep it may be advisable to politely refuse an interview or statement until you can obtain legal advice. Additionally, it is likely best to decline to surrender a controlled substances registration prior to speaking with a lawyer. Giving a statement or surrendering a controlled substances registration can cause irreparable harm to an individual’s case that even an experienced defense attorney might later find it difficult to overcome.