Yohimbine is listed as a prescription drug in Schedule F Part 1 of the memorandum related to this:
Importation of Human-use drugs:
If you look through these documents, you will see in the first category in Appendix A there is a provision for over the counter products provided you are carrying the 3 month or less supply and you are an adult. Shipment is a valid importation method for this category.
Unfortunately yohimbine is considered a prescription drug as shown in Appendix B. Many times customs officers apply the first criterea from Appendix A which explains the allowed shipments and in-person imports.
I was caught last year with it and was let go with a fine (for not declaring), but they let me keep the HOT-ROX Extreme I had - 3 bottles.
Prescription drugs require additional forms of permission including getting your doctor to hook you up. The information for how to go about this can be obtained from references within the memorandum.