development expenses is primarily related to increased salary and benefit expense of $153,000 and consultant expenses of $36,000, partially offset by decreased stock-based compensation expenses of $51,000 and reduced facility allocation expense of $50,000.
Research and development expenses for the nine months ended September 30, 2021 were $3,800,000, an increase of $522,000 compared to the research and development expenses for the nine months ended September 30, 2020 of $3,278,000. The increase in research and development expenses is primarily related to increased salary and benefit expense of $556,000 and increased stock-based compensation and consulting expense of $74,000 and $81,000, respectively, partially offset by reduced facility allocation expense of $177,000.
Sales and Marketing
Sales and marketing expenses for the three months ended September 30, 2021 were $1,021,000, an increase of $186,000, compared to the sales and marketing expenses for the three months ended September 30, 2020 of $835,000. The increase in sales and marketing expenses is primarily related increased salary and benefit expenses of $73,000, website development of $89,000 and advertising expenses of $87,000, partially offset by decreased stock-based compensation of $110,000.
Sales and marketing expenses for the nine months ended September 30, 2021 were $2,870,000, an increase of $827,000, compared to the sales and marketing expenses for the nine months ended September 30, 2020 of $2,043,000. The increase in sales and marketing expenses is primarily related increased salary, incentive compensation and benefit expenses of $233,000, website development and advertising expenses of $272,000 and $370,000, respectively.
General and Administrative
General and administrative expenses for the three months ended September 30, 2021 were $1,081,000, an increase of $67,000, compared to the general and administrative expenses for the three months ended September 30, 2020 of $1,014,000. The increase in general and administrative expenses is primarily related to increased salary and benefit expenses, board fees and investor relations expenses of $22,000, $33,000 and $34,000, respectively, offset partially by increased stock-based compensation of $153,000.
General and administrative expenses for the nine months ended September 30, 2021 were $3,037,000, an increase of $525,000, compared to the general and administrative expenses for the nine months ended September 30, 2020 of $2,512,000. The increase in general and administrative expenses is primarily related to increased salary, incentive compensation and benefit expenses of $178,000, increased stock-based compensation, board fees and investor relations expenses of $87,000, $33,000 and $141,000, respectively, partially offset by a $237,000 expense recorded in 2020 pursuant to the Alexander Settlement Agreement discussed in Note 6 – Convertible Preferred Stock and Stockholders’ Equity (Deficit) recorded in the prior year.
Interest Expense
Interest expense for the three months ended September 30, 2021 was $3,000, a decrease of $2,000 compared to the interest expense for the three months ended September 30, 2020 of $5,000. Minimal interest expense for both periods was primarily related to interest accrued on the PPP loan.
Interest expense for the nine months ended September 30, 2021 was $9,000, a decrease of $1,385,000, compared to the interest expense for the nine months ended September 30, 2020 of $1,394,000. Minimal interest expense recorded in the nine months ended September 30, 2021, was primarily related to interest accrued on the PPP loan. Interest expense for the nine months ended September 30, 2020 was primarily due to the full amortization of debt discounts associated with the convertible debt that the Company incurred in March 2020, as such convertible debt was fully repaid in April 2020.
Change in Fair Value of Warrant Liability
There was no change in the fair value of the warrant liability for the three months ended September 30, 2021 and 2020.
There was no change in the fair value of the warrant liability for the nine months ended September 30, 2021, compared to a gain of $24,000 due to the change in fair value of the warrant liability for the nine months ended September 30, 2020. The gain for the nine months ended September 30, 2020 was due to the decrease in our common stock price during the period.