Thank you for your question. It is not recommended to use a different brush than the one provided, the applicators in the box were designed for the product and ensure you are despensing the appropriate amount for best results. An eyeshadow applicator may absorb to much of the product. Best wishes,Pablo Prichard MD
The Latisse applicator was specifically designed for applying Latisse. That said, if you're able to carefully apply in a similar manner with a different brush which doesn't absorb too much product (wasteful/costly), then there should be no issue.