ANTIFERMENTER for drilling (EN: antifermenter; DE: Antifermentator; FR: antifermentateur; ES: antifermentator; RU: антиферментатор) is the substance, which prevents or decreases the enzymatic decomposition of the starch-containing reagents, of dextran and other polysaccharides, which are added into the drilling fluids.

The strongest (highly toxic) antifermenters are the cetylpyridinium chloride, diocide (the mixture of the bromous N-cetylpyridinium and ethanol-mercury-chloride), catapine (para-dodecyl-methyl-benzyl-pyridinium chloride). They use as antifermenters also the formaldehyde, paraformaldehyde, phenol, phenol, and their derivatives. To certain degree, the prevention of enzymatic degradation of starch is achieved by its alkaline processing with the increasing of pH of the drilling fluid to 11.5-12, by the addition of the sodium chloride to the concentration of 320 grams per litre.

The introduction of antifermenters into the drilling fluids does not ensure the long-time stability of reagents, which are contained there, which are subject to bacterial decomposition, especially at the increased temperature, because the organisms, which are causing bacterial decomposition, acquire immunity. Thus they periodically change antifermenters. The accumulation of antifermenters within the drilling fluid causes the deterioration of its properties. During the usage of the starch reagents, the content of antifermenters should not exceed 1.2-2.4 litres per cubic metre.