Carrier oils play a central role in the formulation of oil-based injectable compounds. Their physicochemical properties determine not only how efficiently the active pharmaceutical ingredient (API) dissolves, but also how the final product behaves upon administration.
This article reviews the fundamental characteristics of carrier oils, compares the major types used in injectable preparations, and explains how these properties influence injection tolerance and long-term stability.
What carrier oils are and why they matter
Carrier oils serve as the vehicle for dissolving lipophilic active substances that cannot be suspended or delivered effectively in water-based formulations. Unlike aqueous suspensions, which rely on micronized solid particles, oil-based solutions provide more consistent release kinetics and improved chemical stability for many steroidal and hormonal compounds.
The choice of oil directly affects solubility, oxidative resistance, filtration compatibility, and overall biocompatibility. As a result, the selection of carrier oil is a critical formulation parameter that determines tolerance, potency retention, and shelf stability.
How carrier oils influence injection tolerance
Injection tolerance varies significantly depending on the physicochemical behavior of the vehicle. Several factors contribute to whether an injection is perceived as smooth or uncomfortable:
- Viscosity — directly affects tissue dispersion; thick oils disperse slowly and increase mechanical irritation.
- Allergenic potential — some plant-derived oils produce immune-mediated reactions that manifest as inflammation or swelling.
- Residual solvent content — insufficient post-synthesis purification can lead to irritation independent of the oil.
- Injection volume and concentration — higher volumes require greater intramuscular accommodation, which can intensify discomfort.
- Spreadability and tissue compatibility — oils differ in how quickly they integrate with surrounding tissue fluids.
MCT/MCP-based oils generally produce lower post-injection pain due to their low viscosity and rapid dispersion. In contrast, highly viscous oils such as castor or sesame oil are more often associated with prolonged post-injection soreness.
Major carrier oils used in injectable formulations
Cottonseed oil
Cottonseed oil is one of the most widely used vehicles in mass-produced injectables due to its balanced physicochemical profile. It has moderate viscosity, good oxidative stability, and a low rate of allergic reactions. These characteristics make it suitable for solutions requiring predictable flow properties and relatively high patient tolerance.
Sesame oil
Sesame oil exhibits higher viscosity and thus may increase injection discomfort when used at larger volumes. However, it provides excellent oxidative stability owing to its natural antioxidant components. Its main limitation is a comparatively higher allergenic potential.
Castor oil
Castor oil has very high viscosity and is used primarily in formulations designed for slow, depot-like release. Its high thickness reduces dispersion speed and is associated with elevated rates of post-injection discomfort. These characteristics make it suitable for specialized long-acting preparations rather than general applications.
Medium-chain carrier oils (MCT/MCP)
Medium-chain triglycerides and propionate-based carriers have become increasingly preferred in modern formulations. They exhibit low viscosity, high solubility for many lipophilic compounds, and rapid dispersion in muscle tissue. MCT/MCP-based vehicles are often associated with improved injection tolerance and reduced post-injection pain. Several pharmaceutical manufacturers, including major endocrine therapy producers, employ MCT-type carriers for these reasons.
Comparison of Common Carrier Oils
| Carrier Oil | Viscosity | Oxidative Stability | Irritation Potential | Typical Use-Cases |
| Cottonseed Oil | Moderate | Good | Low | General injectables; balanced formulations |
| Sesame Oil | High | Excellent | Moderate–High | Stable long-shelf-life preparations |
| Castor Oil | Very High | Good | High | Depot and slow-release formulations |
| MCT/MCP Oils | Low | Moderate | Very Low | Modern injectables; high-tolerance solutions |
Conclusion
Carrier oils are not passive excipients; they are active determinants of formulation stability, solubility, and injection tolerance. Viscosity, oxidative resistance, and solubility capacity collectively shape the pharmacotechnical properties of injectable preparations. Among available options, MCT/MCP oils represent the most modern and well-tolerated carriers, offering rapid dispersion and minimal tissue irritation. In contrast, water-based suspensions behave fundamentally differently, often producing higher irritation and less predictable release profiles.
Understanding these distinctions is essential for the development of stable, tolerable, and pharmacologically reliable injectable formulations.