Electronic cigarette cartridges and liquids which contain nicotine are classified as medicinal products and are regulated under the Japanese pharmaceutical affairs law.
If you wish to bring them into Japan, the amount that can be cleared by the customs is up to 120 ml (liquid) for the one-month dosage. If you wish to import cartridge or liquid beyond this amount, it is necessary to obtain a medicine supervision certificate before travel.
Electronic cigarette cartridges and devices for atomizing liquids are also regarded as medical devices. When importing these, only one device (or two if a spare one is required) can be cleared through the customs clearance.