Loan interest deduction

Investment interest is deductible, with several conditions.

You can only deduct investment interest up to the amount of income you report (dividends or capital gains).  If you have interest more than your current year income, you will track it on form 4952 and it can be carried forward and used as a deduction in the future when you do have income from your investments.

Investment interest is a schedule A itemized deduction, so you won't get a tax benefit unless your total itemized deductions is more than your standard deduction.

You have to follow the tracing rules and be able to allocate your interest to your particular investments.  For example, if you use 100% of the loan proceeds for investment, then 100% of your interest can be allocated to the investments.  But if you use the loan proceeds for other purposes (debt consolidation, vacation, home repair, etc) then it becomes much harder to trace the interest back to the investment income (or at least, much harder to convince an auditor if you get audited).

https://www.irs.gov/forms-pubs/about-form-4952

https://www.irs.gov/forms-pubs/about-publication-550

Other investment expenses, like broker fees and commissions, that used to be miscellaneous itemized deductions subject to the 2% rule, are NOT deductible for tax years 2018-2025 due to the tax reform law.  Those expenses will become deductible again in 2026 unless Congress changes the rules again.