Interest paid on margin loans and other debt incurred to purchase investment assets - stocks, bonds, mutual funds - is deductible, but only up to the amount of net investment income for the year. Unlike business interest (which is deductible against business income) or mortgage interest (which is deductible against ordinary income subject to limits), investment interest expense is ring-fenced: it can only offset investment income. Disallowed investment interest carries forward indefinitely and is deductible in future years when the taxpayer has sufficient net investment income. For active investors using margin heavily, this limitation can create a significant tax timing difference.
What qualifies as investment interest: Interest on margin loans from a broker, interest on loans used to purchase taxable bonds or dividend-paying stocks, and interest on loans to purchase passive investment real estate (but only if not already covered by §469). Does not include interest on loans to purchase tax-exempt bonds (§265 disallows that deduction entirely).
Net investment income: Gross investment income (interest, ordinary dividends, short-term capital gains, royalties, annuity income from investments) minus investment expenses. Long-term capital gains and qualified dividends are NOT investment income by default - they receive preferential rates precisely because they are treated separately from ordinary investment income.
The limit: Deductible investment interest = the lesser of actual investment interest paid OR net investment income. Any excess is carried forward to the next year with no expiration.
Taxpayers may elect under §163(d)(4)(B) to treat net capital gain (long-term capital gains) and qualified dividends as investment income for purposes of the §163(d) limitation. This election increases the investment income ceiling and allows a larger current deduction for investment interest expense - but at a cost. The income included in the election loses its preferential capital gain / qualified dividend tax rate for that year and is taxed as ordinary income.
When the election makes sense: a taxpayer with $50,000 of margin interest and $60,000 of net investment income (including $40,000 of long-term gains elected as investment income) can deduct the full $50,000. Without the election, only $20,000 of investment income is available and only $20,000 is deductible currently. The election costs the preferential rate on $40,000 of gains (potentially $8,000 of additional tax at the spread between 37% and 20%) but frees $30,000 of additional current deduction (worth $11,100 at 37%). The math favors the election in this example.
Investment interest expense deduction is calculated on Form 4952 (Investment Interest Expense Deduction) and flows to Schedule A as an itemized deduction. The form tracks: total investment interest paid, net investment income, the current year deduction, and the carryforward amount. Because it is an itemized deduction, it only produces a tax benefit if total itemized deductions exceed the standard deduction. Taxpayers who take the standard deduction receive no benefit from investment interest expense in that year - but the carryforward continues for future years when they may itemize.
Margin loans against a taxable brokerage account allow investors to borrow against their securities without selling (and without triggering capital gains). The interest is potentially deductible (subject to §163(d)). This creates a "buy, borrow, die" strategy for wealthy investors: buy appreciated assets, borrow against them tax-free (loans are not income), deduct the interest against investment income, and at death the assets receive a step-up in basis that eliminates the embedded gain. The IRS has been attentive to this strategy, but the underlying tax mechanics are well-established law.