Menthol crystals. Use sparingly as these menthol crystals are very strong. A little goes a long way! You may also make your own menthol drops by placing a few grams of crystals into an alcohol base such as ethanol (vodka).
TIP: To dissolve, fill a container half way with VG or PG, then the other half with menthol crystals. Shake until dissolved. Heating the VG or PG first will speed up the process. Try starting out at 5% in your recipe of that solution that you made.
*NOTE: Heat will melt menthol crystals. You menthol crystals may melt during shipping, especially during the summer months and areas where the climate is hot. This does not harm the menthol in anyway. You can turn it back into crystals by tapping it with a hammer or other hard object.