An excess of water in herbal materials encourages microbial growth, presence of fungi or insects and deterioration following hydrolysis. At suitable temperatures, enzymes are activated leading to hydrolytic degradation of active constituents (e.g. digitalis). As most vegetable drugs contain all the food requirements for the growth of moulds, insects and mites, deterioration is very rapid once infestation takes place. Limits for water content should therefore be set for every herbal material. This is especially important for drugs that absorb moisture easily or deteriorate quickly in the presence of moisture. Many methods are now available for moisture content determination.

The test for loss on drying employed in British Pharmacopolia (BP), Extra Pharmacopolia (EP), United States Pharmacopolia (USP) and Indian Pharmacopolia (IP) determines both water and volatile matter. Drying can be carried out either by heating to 100–105° C or in a desiccator (spreading thin layers of weighed drugs over glass plates) over phosphorous pentoxide R under atmospheric or reduced pressure at room temperature for a specified period of time. The desiccator method is especially useful for materials that contain considerable proportion of volatile matter and/or those that melt to a sticky mass at elevated temperatures (balsams and resins). For materials such as digitalis, aloes, starch and fibres which contain little volatile matter, direct drying to constant weight can be employed. Use of moisture balance that combines both drying process and weight recording is suitable where large numbers of samples are handled and where a continuous record of loss in weight with time is required.

The azeotropic method of moisture determination gives a direct measurement of the water present in the material being examined. When the sample is distilled together with an immiscible solvent such as toluene, xylene or carbon tetrachloride, the water present in the sample is absorbed by the solvent. The water and solvent are distilled together and separated in the receiving tube on cooling. To avoid water remaining absorbed in the solvent even after distillation, the solvent is saturated with water before use.

Chemical method of moisture content determination using Karl Fischer reagent is employed for expensive drugs and those which contain small quantities of moisture. Dry extracts of alkaloid-containing drugs, alginic acid and fixed oils such as arachis, castor oil and sesame oil for parenteral use are usually evaluated by this method. Other chemical methods for water determination include treating the sample with various carbides, nitrides and hydrides and measuring and analysing the gas evolved using GC.