i found a yellow pill CB 400 on it , what is it?

Hi Rocky9960,

I looked up the imprint code "CB 400" in Micromedex, which is a
proprietary database to which I have access through my medical
library.  The description below is for Cimetidine:

CB 400 (IMPRINT CODE): CIMETIDINE COATED TABLET 400 MG from LEDERLE
LABORATORIES
IMPRINT CODE: CB 400
COLOR: LIGHT YELLOW
SHAPE: OVAL
FORM: COATED TABLET
ACTIVE INGREDIENTS:
   CIMETIDINE 400 MG
REGISTRIES: NDC 00005-3417
FOR MORE INFORMATION CONTACT: LEDERLE LABORATORIES

This appears to agree with your description above.  

Cimetidine is the generic name for Tagamet, Zantac, and Pepcid, the
popular anti-ulcer and anti-heartburn drugs.

Here are two pages that describe this drug, both of which I found
using the MEDLINEplus consumer health system drug information
database:

From MedMaster
http://www.nlm.nih.gov/medlineplus/druginfo/medmaster/a682256.html

From the USP DI
http://www.nlm.nih.gov/medlineplus/druginfo/uspdi/202283.html

librariankt

This should answer your question:

http://rxlist.com/cgi/rxlistid.cgi?drug=*cb400&x=9&y=5