The National institute of health just completed a huge study on these
supplements and found that glucosamine and chondroitin alone was not
effective.   There was some indication that glucosamine and
chondroitin might be effective when used together, but that it needed
more study.  
http://nccam.nih.gov/research/results/gait/qa.htm
http://www.arthritis.org/resources/arthritistoday/2006_archives/2006_01_02/2006_
01_02_gait.asp

You might try the two together and if you notice no benefit then stop
taking them.  

You need to discuss the injections with your Dr. -- MZ