Revenues
Total revenue for the six months ended June 30, 2024 was $33.9 million, an increase of 2% compared to the six months ended June 30, 2023 revenue of $33.2 million. During the six months ended June 30, 2024, total U.S. revenue increased by 2% and international revenue increased by 5%. The U.S. revenue growth was primarily driven by an increase in treatment sessions sales. The increase in international revenue was primarily driven by an increase in treatment session sales.
U.S. NeuroStar Advanced Therapy System revenue for the six months ended June 30, 2024 was $7.3 million, a decrease of $1.0 million compared to the six months ended June 30, 2023 revenue of $8.3 million. For the six months ended June 30, 2024 and 2023, the Company sold 89 and 103 systems, respectively, that were recognized as NeuroStar Advanced Therapy System capital revenue during each period. Additionally, for the six months ended June 30, 2024 the Company executed 1 operating lease agreement which contributed to operating lease revenue.
U.S. treatment session revenue for the six months ended June 30, 2024 was $24.6 million, an increase of 7% compared to the six months ended June 30, 2023 revenue of $23.0 million. The revenue growth was primarily driven by an increase in treatment session volume and utilization over the comparative period.
Cost of Revenues and Gross Margin
Cost of revenues decreased by $0.4 million, or 4%, from $9.0 million for the six months ended June 30, 2023 to $8.6 million for the six months ended June 30, 2024. Gross margin increased from 72.9% for the six months ended June 30, 2023 to 74.6% for the six months ended June 30, 2024. The increase in gross margin was primarily a result of increased sales volume of our treatment session sales resulting in a more favorable product mix and absence of one-time manufacturing cost charged in 2023 by our new contract manufacturer.
Sales and Marketing Expenses
Sales and marketing expenses increased by $0.4 million, or 2%, from $23.5 million for the six months ended June 30, 2023 to $23.9 million for the six months ended June 30, 2024. The increase was primarily driven by growth in co-op marketing and bad debt expense.
General and Administrative Expenses
General and administrative expenses decreased by $0.7 million, or 6% from $12.8 million for the six months ended June 30, 2023 to $12.1 million for the six months ended June 30, 2024. The decrease was primarily due to a decrease in personnel expenses and legal costs in the current period versus the prior period.
Research and Development Expenses
Research and development expenses decreased by $0.6 million, or 11% from $5.2 million for the six months ended June 30, 2023 to $4.6 million for the six months ended June 30, 2024. The decrease in research and development was primarily due to higher capitalization of product development costs related to certain of the Company’s software project costs.
Interest Expense
Interest expense increased by $1.4 million from $2.4 million for the six months ended June 30, 2023 to $3.8 million for the six months ended June 30, 2024 due to an increase in interest rates and debt balance.
Other Income, Net
Other income, net decreased by $2.7 million from $4.2 million for the six months ended June 30, 2023 to $1.5 million for the six months ended June 30, 2024, primarily as a result of ERC of $2.9 million recorded for the