periods was driven by our continued growth of WAKIX and the increase in personnel costs for the nine months ended September 30, 2024 was related to increased headcount.
General and Administrative Expenses
General and administrative expenses increased by $6.0 million, or 26.8%, for the three months ended September 30, 2024, and increased by $14.1 million, or 20.9%, for the nine months ended September 30, 2024, compared to the same periods in 2023. The increase in the three months ended September 30, 2024, was primarily due to a $4.0 million increase in legal and professional fees, primarily associated with patent lawsuits, a $2.0 million increase in stock compensation associated with new awards, and a $0.5 million increase in personnel costs. The increase in the nine months ended September 30, 2024, was primarily due to a $7.5 million increase in legal and professional fees, primarily associated with patent lawsuits, a $6.1 million increase in stock compensation associated with new awards, and a $2.0 million increase in personnel costs, partially offset by a $1.3 million decrease in insurance and a $0.5 million decrease in travel expenses.
Interest Expense
Interest expense decreased by $2.7 million, or 38.0%, for the three months ended September 30, 2024, and decreased by $5.7 million, or 29.9%, for the nine months ended September 30, 2024, compared to the same periods in 2023. The decrease for the three and nine months ended September 30, 2024, was primarily due to lower interest rates as a result of refinancing into the TLA Credit Agreement (defined below).
Interest Income
Interest income increased by $0.8 million, or 20.1%, for the three months ended September 30, 2024, and increased by $3.4 million, or 32.2%, for the nine months ended September 30, 2024, compared to the same periods in 2023. The increase for the three and nine months ended September 30, 2024, was primarily a result of having higher invested balances, and higher investment yields on those balances, compared to the prior year periods.
Income Taxes
Income tax expense was $16.1 million, representing a 25.9% effective tax rate, for the three months ended September 30, 2024, compared to $13.4 million, representing a 25.8% effective tax rate, for the three months ended September 30, 2023. Income tax expense was $39.6 million, representing a 29.2% effective tax rate, for the nine months ended September 30, 2024, compared to $31.5 million, representing a 23.5% effective tax rate, for the nine months ended September 30, 2023. The increase in our effective tax rate for the nine months ended September 30, 2024, was primarily driven by the $17.1M IPR&D charge related to the Epygenix acquisition, which was nondeductible for tax purposes. The effective tax rate of 25.9% for the three months ended September 30, 2024, included 5.1% in state income taxes, partially offset by a 0.9% benefit from credits. The effective tax rate of 29.2% for the nine months ended September 30, 2024, included 2.5% for nondeductible IPR&D charges and 6.3% in state income taxes, partially offset by a 1.2% benefit from credits.
Liquidity, Sources of Funding and Capital Resources
Overview
As of September 30, 2024, we had cash, cash equivalents, and investments of $504.7 million, outstanding debt of $185.0 million and an accumulated deficit of $47.3 million.
The unaudited condensed consolidated financial statements have been prepared as though we will continue as a going concern, which contemplates the realization of assets and satisfaction of liabilities in the normal course of business.