Injection refers to the sterile preparation made of raw materials and auxiliary materials for injection into human body. In the early stage of product development, it is necessary to select appropriate solvents and additives according to the characteristics of raw materials to meet the safety, effectiveness and stability of products.

1、Common solvents for injection

（1）Pharmaceutical water

The 2025 edition of China Pharmacopoeia divides pharmaceutical water into drinking water, purified water, water for injection and sterilized water for injection.

1）The raw water for pharmaceutical water is usually drinking water: drinking water is the water obtained by purifying natural water, which can be used as an extraction solvent for important decoction pieces unless otherwise specified.

2）Purified water: Purified water is usually pharmaceutical water prepared by reverse osmosis, ion exchange, distillation or other methods. Purified water does not contain any additives, and can be used as experimental water, solvent for common pharmaceutical preparations, solvent or diluted solution for oral or external preparations. Although the purified water is produced by strict technology, there are still many ions and organic substances, and there are endotoxin and microbial risks, so it should not be used to prepare or dilute injections.

3）Water for injection: The water for injection is distilled from purified water without any additives. It can be used as a solvent or diluent for injections and eye drops, and can also be used as a cleaning solvent for production systems. Water for injection must pass endotoxin and microbial examination.

4）Sterilized water for injection: Sterilized water for injection is often used as a solvent for sterile powder for injection or a diluent for injection, and is generally managed according to the approval number of drugs.

（2）Oil for injection

Some raw materials are hydrophobic and insoluble in water, and oil for injection is often used as solvent. Commonly used oils for injection include sesame oil, tea oil, soybean oil, peanut oil, corn oil, cottonseed oil, olive oil, etc. After refining, they can be used for injection. It should be noted that the quality of oil for injection should meet the requirements of China Pharmacopoeia.

（3）Other solvents for injection

Besides water and oil for injection, propylene glycol, polyethylene glycol, glycerol and glycerol can also be used as injection solvents.

1）Ethanol: The concentration of ethanol as injection solvent can reach 50%, and it can be mixed with water, glycerin, etc. For example, hydrocortisone injection contains ethanol, which can be injected intravenously or intramuscularly, but if the ethanol concentration is too high, it may cause hemolysis or pain, and it is generally not recommended to exceed 10%.

2）Propylene glycol: Propylene glycol is often mixed with other solvents (water, ethanol, glycerol) as a composite solvent, containing 10%-60%, which has been widely used in injections. For example, phenytoin sodium injection for epilepsy contains 40% propylene glycol, and solvents containing propylene glycol are generally used for intramuscular injection and intravenous injection. However, intramuscular injection and subcutaneous injection will have local irritation.

3）Polyethylene glycol: Polyethylene glycol has good chemical stability, and both polyethylene glycol 300 and polyethylene glycol 400 can be used as injection solvents. However, there are reports that the degradation products of polyethylene glycol 300 may cause kidney diseases, so polyethylene glycol 400 is more commonly used. For example, Cetepa injection uses polyethylene glycol 400 as a solvent.

4）Glycerol: Glycerol, also known as glycerol, can be mixed with water or alcohol, but it is insoluble in fatty oil and volatile oil. Glycerol itself has high viscosity and irritation, so it is not used as a solvent for injection alone, and its common concentration is 1%-50%. For example, the solvent of procaine injection is a compound solvent of glycerol: 95% ethanol accounts for 20%, glycerol accounts for 20%, and water for injection accounts for 60%.

2、Additive for injection

In addition to the main drug, other substances need to be added to the injection according to the requirements of preparation, storage, transportation and use, so as to increase the stability, safety and effectiveness of the injection. These substances are collectively called additives for injection.

Additives are mainly used in the following aspects: ① to increase the stability of drugs; ② increasing the solubility of drugs; ③ Regulating the osmotic pressure of drugs; (4) For the multi-dose products that need to control microorganisms, the antibacterial performance should also be taken into account; ⑤ Relieve pain or irritation; ⑥ Adjust pH; ⑦ For freeze-dried products, adding additives can also help to improve the appearance and the re-dissolution efficiency.

It should be noted that the concentration of additives should be chosen to be non-toxic, and there is no incompatibility between the main drug and other accessories in the product, which will not affect the content determination of the main drug and the curative effect of the product.

Based on literature review and personal experience, the author combed the commonly used additives for injection and their concentration, so as to provide reference for R&D colleagues. Common additives are shown in the table below.

References:

[1]China Pharmacopoeia (2025 edition)

[2]Pharmacy (People's Health Publishing House, 2023)

[3]Insoluble pharmaceutical preparation technology (CRC Press, USA)

[4]Pharmaceutical expertise (China Medical Science and Technology Press)

Disclaimer: This article is only for the purpose of knowledge exchange and sharing and popular science, and does not involve commercial propaganda, and is not used as relevant medical guidance or medication advice. If the article is infringing, please contact to delete it.

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