The concept of the effective growth of the money supply is introduced, on the basis of the quantity theory of money. That is M2 growth with correction for real GDP growth and changes in the velocity of money circulation. Theoretically, this indicator should be exactly equal to the GDP deflator, and the GDP deflator should be equal to inflation, however, in practice, these indicators differ. A panel study of annual indicators of the GDP deflator, inflation, and effective growth of the money supply is carried out on the example of Russia and the United States. It is shown that, despite the radical differences in the financial systems of Russia and the United States, as well as different inflation rates, the main causes of inflation are monetary. The possible impact on the financial stability of the use of cryptocurrencies is analyzed. It is shown that their use as exotic investment instruments with high risk is permissible, but subject to the control of regulators. Only the emergence of stable cryptocurrencies with the possibility of settlements is unacceptable, as this will create a direct threat to financial stability due to uncontrolled money supply growth.