Patents, net consisted of the following (rounded to nearest thousand):

The patents are amortized on a straight-line basis over the useful lives of the assets, determined to be 12-17 years from the date of acquisition.

Amortization expense for the three months ended December 31, 2019 and 2018 was approximately $93,000 and $92,000, respectively and was approximately $186,000, and $185,000 for the six months ended December 31, 2019 and 2018, respectively.

During the six months ended December 31, 2019 and 2018, the Company has written off the Prurisol patent and other patents of approximately $0 and $155,000, respectively and included in general and administrative expenses.

At December 31, 2019, the future amortization period for all patents was approximately 5.68 years to 16.75 years. Future estimated annual amortization expenses are approximately $186,000 for the year ending June 30, 2020, $372,000 for each year from 2021 to 2024, and total of $1,527,000 for the year ending June 30, 2025 and thereafter.