The determination of matter extractable into solvents such as water, alcohol and ether is generally used for evaluating plant drugs, the constituents of which are not readily estimated by chemical or biological methods. Extractive values thus provide assay for drugs such as linseed, which contain fixed oil as an important constituent. Here ether-soluble extractive is required to fall within the prescribed range. Similarly water-soluble extractive values for drugs like liquorice, valerian and gentian provide an assay process. Yield of drugs to solvent which normally do not have much action upon the drug itself may be used to detect and approximately determine the amount of an adulterant which dissolves in the solvent. For example, when colocynth seeds are used with the pulp in powdered colocynth, its petroleum ether-soluble extractive is higher than normal. On the other hand the amount of insoluble matter will also indicate the presence of an unreasonable amount of woody matter or pieces of bark or vegetable debris in drugs such as myrrh, balsam of tolu, catechu, benzoin etc. However when assay methods are developed, e.g., assay for anthraquinone drugs, the extractive tests are no longer required as pharmacopoeial standards.

Extraction of drugs is by maceration or by percolation. Water, ethanol, ether and petroleum ether are the usual solvents used. Concentration of ethanol specified in the test procedure for the herbal material is used for extraction (90% ethanol for ginger and jalap, 60% ethanol for valerian and 45% ethanol for quillaia).