A DEA number is a number assigned to a health care provider (such as a medical practitioner, dentist, or veterinarian) by the U.S. Drug Enforcement Administration allowing them to write prescriptions for controlled substances. Legally, the DEA number is solely to be used for tracking controlled substances. It is often used by the industry, however, as a general "prescriber number" that is a unique identifier for anyone who can prescribe medication.
A valid DEA number consists of:
- 2 letters, 6 numbers, & 1 check digit
- The first letter is a code identifying the type of registrant (see below)
- The second letter is the first letter of the registrant's last name
- Of the seven digits that follow, the seventh digit is a "checksum" that is calculated as:
- Add together the first, third and fifth digits call this CALC1,3,5
- Add together the second, fourth and sixth digits and multiply the sum by 2, call this CALC2,4,6
- Add CALC1,3,5 + CALC2,4,6 call this CHECK
- The rightmost digit of CHECK (the digit in the ones place) is used as the check digit in the DEA number
Registrant type (first letter of DEA Number):
- A/B/F/G - Hospital/Clinic/Practitioner/Teaching Institution/Pharmacy
- M - Mid-Level Practitioner (APN/CNP/PA/OD/ET,etc.)
- P/R - Manufacturer/Distributor/Researcher/Analytical Lab/Importer/Exporter/Reverse Distributor/Narcotic Treatment Program
Prior to October 1, 1985, DEA registration numbers for physicians, dentists, veterinarians, and other practitioners started with the letter A. New registration numbers issued to practitioners after that date begin with the letter B, F, or G. 
Per US DOJ: "Due to the large Type A (Practitioner) registrant population, the initial alpha letter "B" has been exhausted. DEA will begin using the new alpha letter "F" as the initial character for all new registration for Type A (Practitioner) registrations."