What characteristic is essential for substances that are injected subcutaneously?

For substances that are injected subcutaneously, it is essential that they are sterile and properly packaged. This characteristic ensures that the medication does not introduce any pathogens into the body, thus reducing the risk of infection at the injection site. Sterility is crucial because subcutaneous injections bypass the body's natural barriers, allowing for direct entry into the bloodstream and tissues.

Proper packaging also plays a key role in maintaining the sterility of the substance, protecting it from contamination before administration. While the composition of the substance is important, especially in terms of solubility and absorption, the primary concern for any injectable medication is that it maintains sterility to ensure patient safety.

The other options do not pertain to the fundamental requirements for subcutaneous injections. For example, having active ingredients is necessary for effectiveness, but they do not affect the safety of the injection process itself. Similarly, the route of administration does not require medications to be rich in fat or blood vessels, nor must they be in tablet form, as such formulations are not suitable for subcutaneous injections.