The quality of medicines is generally controlled through the observation of the physical properties of the medicines, the determination of physical constants, the inspection of impurities and their limits, and the determination of their content. The impurity of a drug is a substance in a drug that has no therapeutic effect, or is harmful to the human body or affects the quality of the drug. The impurity inspection method is an important indicator to control the quality of the drug. Whether a drug is good and effective should not be judged solely from the content of the main components, but should be assessed from various aspects. Not only the efficacy of the drug itself and its side effects should be considered first, but also the comprehensive evaluation of excipients, other coexisting drugs or possible impurities, as well as their degree, measure and impact on the human body.

Of course, the control of drug purity cannot be simply understood as the items and limits of impurities, but should be comprehensively and comprehensively examined by considering the various indicators of drug quality as a whole. From the perspective of the content of impurities, the less the better, but from the perspective of the method of removing impurities and economic considerations, if the method of removing impurities is complicated and the operation is difficult, it will inevitably lead to increased production costs and hinder the development of production. Therefore, under the principle of not affecting the efficacy, preparation, storage and human health, the limit of impurities contained in the drug should be as far as possible so as not to affect the quality. And adhere to strict requirements and high standards, gradually improve the production process, improve the quality of drugs, reduce the content of impurities, and catch up with the international advanced level.

Due to the complex production process of drugs and different sources of raw materials, the impurities in intermediates, intermediate products and raw materials may be various, and it is difficult to inspect or control them one by one. Check for impurities to control it. For example, sulfuric acid and hydrochloric acid are commonly used as raw materials in the production process of many medicines, so chlorides and sulfates are most likely to be mixed in medicines. Many varieties in the Pharmacopoeia stipulate that chloride or sulfate must be checked. The inspection method is simple and the sensitivity is very high. If the chloride and sulfate contained in the control are below a certain limit, some cationic groups combined with chloride ion or sulfate ion will be affected. Some by-products generated at the same time can also be controlled relatively below a certain limit. In fact, a small amount of chlorides and sulfates themselves are not toxic and have no effect on the human body, and the Pharmacopoeia defines them with wider limits, because the inspection of chlorides and sulfates, the control of other impurities has a special The meaning of , can show the purity of the drug from their situation, so they are called "indicative impurities". If the limits for chlorides and sulfates are relaxed, the limits for other impurities are also relaxed.

Another example is some organic drugs. The Pharmacopoeia stipulates that the residues on ignition and easy carbides should not exceed a certain limit, so as to control the organic impurities that may be mixed with inorganic impurities and concentrated sulfuric acid and cause carbonization, so that there is no need to check them one by one. What exactly are these inorganic and organic impurities. At present, the inspection of special impurities mostly adopts chromatography, spectroscopy and so on.

Whether the drug contains raw materials, intermediates and by-products used in the manufacturing process must also be controlled by the impurity inspection items, such as the inspection of free hydrazine in isoniazid. Because some of these substances are harmful to human health, some affect the determination of content.

Highly toxic substances should be controlled with stricter limits. If arsenic is toxic to humans, the pharmacopoeia stipulates that its limit generally does not exceed ten parts per million (10ppm). Heavy metals (referring to metal impurities that can react with hydrogen sulfide to precipitate under weak acidity) are easy to accumulate in the body, cause chronic poisoning, and cause great harm to the human body. ).

Whether the drug is decomposed during storage should be controlled through the inspection of impurities, because if the drug is decomposed, it will not only fail, but also may be harmful to the human body. For example, acetylsalicylic acid (aspirin) is easily affected by moisture during storage, and hydrolyzes to generate salicylic acid and acetic acid. These products are both irritating to the stomach and do not have the antipyretic and analgesic properties of acetylsalicylic acid. Therefore, free salicylic acid should be controlled.

There may be several manufacturing methods for drugs, the specifications of the raw materials used are sometimes different, and the equipment and technical conditions of each factory are also inconsistent. Therefore, the regulations on the types and limits of drug impurities should be closely combined with the actual production. Appropriate provisions should be made according to the raw materials used, the production process and the products that may decompose during storage. Impurities that endanger people's health and affect the stability of finished products must be strictly controlled; the inspection and limit regulations of impurities should not only reflect the technical level of drug production in my country, but also be conducive to the development of production.

The inspection items specified in the drug standard are formulated according to the impurities that may be brought in in general production. Therefore, if the production process is changed, the inspection items should be changed accordingly. It should also be emphasized that some impurities that are not specified in the drug standard program do not mean their absence. Obviously, soluble barium salts are not allowed in raw drug products for intravenous injection, but the Pharmacopoeia does not stipulate that every drug product must be checked for barium salts.

Although general chemicals and chemical reagents also have purity standards, these purity regulations are based on impurities and their maximum limits to suit different purposes, and do not consider the physiological effects of impurities. Therefore, the drug quality supervision department stipulates that general Chemicals and chemical reagents replace pharmaceutical specifications.