The serial number always starts out with an A or B followed by another letter. The first letter is A for physicians who got their numbers before approximately 1980, thereafter they begin with B. (This may not be a hard and fast rule, but it seems generally true.) The next letter is simply the first letter of the doctor's last name! So old Dr. Smith's number will begin AS.
Next comes a seven digit registration number. The final digit of this number is called the "check digit." The "check digit" is used to verify the rest of the number by processing it with the proper algorithm.
In the case of the DEA registration number, the algorithm is as follows: the "check digit" will equal the last digit of the sum of a) the addition of the 2nd, 4th, and 6th digits times two PLUS b) the sum of the addition of the 1st, 3rd, and 5th digits. That sounds confusing, but as far as algorithms go, it's kid's stuff.
Let's look at an example: You've made a prescription pad for old Dr. Smith, so your letters are gonna be AS. Now make up a six digit number and write it down. It does not matter what it is. Let's say it's 375987. You create your "check digit" by adding the 2nd, 4th, and 6th digits (7+9+7)--the sum is 23. Multiply that by 2 and you get 46. Now add the 1st, 3rd, and 5th digits [3+5+ 8] --the sum is 16. Add 46 and 16 and you get 62. The last digit of 62 is 2, so that's your "check digit" and your full DEA registration is AS3759872. Get a piece of paper and do this for yourself. It will make sense.
[ 08 August 2002: Message edited by: fairnymph ]