In the Affordable Care Act, the new tax on investment income is called an “unearned income Medicare contribution.” However, the law does not provide for the money to be deposited in a specific trust fund. It is added to the government’s general tax revenues and can be used for education, law enforcement, farm subsidies or other purposes.

Donald B. Marron Jr., the director of the Tax Policy Center, a joint venture of the Urban Institute and the Brookings Institution, said the burden of this tax would be borne by the most affluent taxpayers, with about 85 percent of the revenue coming from 1 percent of taxpayers. By contrast, the biggest potential beneficiaries of the law include people with modest incomes who will receive Medicaid coverage or federal subsidies to buy private insurance.

Wealthy people and their tax advisers are already looking for ways to minimize the impact of the investment tax — for example, by selling stocks and bonds this year to avoid the higher tax rates in 2013.

The new 3.8 percent tax applies to the net investment income of certain high-income taxpayers, those with modified adjusted gross incomes above $200,000 for single taxpayers and $250,000 for couples filing jointly.

David J. Kautter, the director of the Kogod Tax Center at American University, offered this example. In 2013, John earns $160,000, and his wife, Jane, earns $200,000. They have some investments, earn $5,000 in dividends and sell some long-held stock for a gain of $40,000, so their investment income is $45,000. They owe 3.8 percent of that amount, or $1,710, in the new investment tax. And they owe $990 in additional payroll tax.

The new tax on unearned income would come on top of other tax increases that might occur automatically next year if President Obama and Congress cannot reach an agreement in talks on the federal deficit and debt. If Congress does nothing, the tax rate on long-term capital gains, now 15 percent, will rise to 20 percent in January. Dividends will be treated as ordinary income and taxed at a maximum rate of 39.6 percent, up from the current 15 percent rate for most dividends.

Under another provision of the health care law, consumers may find it more difficult to obtain a tax break for medical expenses.