The breakdown of the oily vitamins or the oxidation of vitamin C may be generating CO₂ as a byproduct, especially if the solution is exposed to light, heat, or oxygen. Here are a few strategies to mitigate this issue:

Use Antioxidants: Incorporating antioxidants like tocopherols (vitamin E) or ascorbyl palmitate can slow the oxidation of vitamin C and the oily vitamins, reducing CO₂ buildup.

Modify Storage Conditions: Store the solution in a cooler, dark environment to reduce the rate of oxidation. Light and heat can accelerate the decomposition of vitamins, leading to gas formation.

Improve Container Sealing: Consider using containers with better gas-permeability control. Some modified polypropylene containers or those with gas-permeable membranes might allow slow release of CO₂, preventing pressure buildup.

Buffered Formulation: Adjusting the pH with a suitable buffer could stabilize the vitamin C, reducing its decomposition into CO₂. However, ensure the buffer is compatible with both vitamin C and the oily vitamins.

Use Amber or Opaque Containers: These can help protect the solution from light-induced degradation, reducing the risk of gas formation.