It's referenced in a lot of places yet I can't find it.  It's suppose
to be on the FDA site.  Simply put, all prescripton drugs fall into a
Therapeutic Class.  I just want the official government list of all
these classes.
I've tried the following links and it does NOT give me the list of
Therapeutic Classes http://www.fda.gov/cder/ndc/database/default.htm

Hi gary1960-ga,

Good day and thanks for your question. I visited the FDA web site
[http://www.fda.gov/cder/ndc/index.htm] and section 9 caught my
attention:
"The Major and Minor Drug Class codes are no longer available at this
site. FDA plans to review the use of the AMA DRUG Evaluation
Subscription classification scheme. This review is necessary because
this classification scheme has not been updated since 1976 and
therefore many new molecular entities are not included."

The major drug class is a general therapeutic or pharmacological
classification scheme for drug products reported to the FDA under the
provisions of the Drug Listing Act.

On further research, I found out:
[Source: http://www.cdc.gov/nchs/data/nhanes/nhanes_01_02/rxq_rx_b_doc.pdf]
"Since the creation of the NHANES prescription medication file, the
Food and Drug Administration (FDA) has withdrawn the National Drug
Code (NDC) Directory?s therapeutic class codes from their internet
site (http://www.fda.gov/cder/ndc/index.htm)"

I then searched for the older data files (2002 was when they withdrew
the codes), and came across the following:
http://www.fda.gov/cder/ndc/tbldclas.txt (which was updated in April
2005) and contains the 159 class codes.

I hope this answered your question. If you have any questions, please
don't hesitate to request a clarification. I will be more than happy
to clarify your answer.

Thanks for using this service and have a nice day.

Regards,
aditya2k-ga

---

Clarification of Answer by aditya2k-ga on 19 Jun 2005 18:55 PDT

Hey,

Thanks for the rating. The old list *is* on the FDA site -
http://www.fda.gov/cder/ndc/tbldclas.txt (as mentioned in the answer).
It's not linked to anymore.

-aditya2k

Thanks. This explains a lot.  I thought the old list might be around
somewhere but obviously not on the FDA site.

This is the $60,000 question.  I can shed a little more light.
Because of Medicare's Part D program, there is now a generally
accepted list of therapeutic categories and pharmacologic classes for
drugs.  This was developed by USP for CMS.
It would be fairly handy if FDA decided in their efforts to start
here, wouldn't it?
I can't provide a link (their site forbids it) but you can google "USP
Model Guidelines" and you'll get the doc you are looking for.
So, this gives you a decent guideline which maps from Class to drug. 
Next step is to go from Drug to NDC, but that does not appear to be
available.
You may also find the NIH RxNorm project interesting.  This is a big
scary database, and it does not yet include the Class to NDC mapping,
but we can all hope.