A natural starch derivative. CMS is used primarily for fluid-loss control in drilling muds, drill-in, completion, and workover fluids. It is slightly anionic and can be affected by hardness and other electrolytes in a mud. CMS is similar to CMC (carboxymethylcellulose) in method of manufacture and many of its uses. The linear and branched starch polymers in natural starch react with monochloroacetic acid in alkaline solution, adding carboxymethyl groups at the OH positions by an ether linkage. By adding the carboxymethyl groups, the starch becomes more resistant to thermal degradation and bacterial attack.