According to China's foreign exchange management regulations, it is forbidden to use foreign exchange for private transactions. Therefore, it is illegal to buy supplements with foreign exchange. In addition, it is illegal to buy supplements in cash, because cash is also foreign exchange, and foreign exchange can only be legally purchased through banks or financial institutions.

Cash refers to various payment vouchers expressed in foreign currency, which can be circulated and transferred in the international market, and can also be freely converted into foreign currencies of other countries.