A lot of people will bleed a little bit under the new tax law just passed in Washington. A few people -- most of them wealthy -- will bleed a lot. Among the changes affecting your tax returns and your tax planning:

* Medical deductions. They'll be much smaller for the 31 percent of people who itemize deductions. "In many cases," according to the tax experts at Prentice Hall, "the new law wipes out the deduction for all but those taxpayers incurring costs for extraordinary or catastrophic illnesses not covered by adequate medical insurance."

Beginning next year, you may write off only those unreimbursed medical expenses that exceed 5 percent of your adjusted gross income. This year, by contrast, you can deduct all such expenses that exceed 3 percent.

If you're in a position to pick a date for elective surgery, and the bill won't be fully covered by hospital insurance, try to fit your operation into 1982 -- and pay for it -- so that a larger portion of the medical bill will be deductible. Also, try to accelerate other medical expenses into the current year -- for example, physical exams, eye checkups, visits to the dentist and purchases dictated by medical necessity, like an in-home stair elevator. Next year, you'll pay more in real dollars for all your uninsured medical bills.

You'll also lose your right to deduct an extra $150, to help offset the cost of medical insurance. Starting in 1983, insurance premiums will be lumped together with all your other medical expenses, instead of having a separate deduction all their own.

Starting in 1984, the cost of drugs will also be lumped in with other medical expenses. (Right now, money spent on drugs is deductible only to the extent that it exceeds one percent of your income.) But you'll lose your current right to deduct the cost of certain nonprescription drugs. For medical-expense purposes, starting in 1984 the IRS will accept deductions only for prescription drugs and insulin.

* Casualty deductions. For many years, you have been able to tax-deduct each uninsured loss from casualty or theft, to the extent it exceeded $100. This tax subsidy for uninsured losses led many people to reduce the size of their homeowner's and auto-insurance policies. It has been more economical to let Uncle Sam share small losses than to cover them with insurance.

Starting next year, you'll find it hard to qualify for this deduction. Casualty and theft losses will be deductible only if they exceed $100 plus 10 percent of your adjusted gross income. On a $30,000 income, for example, you'll be able to write off only those losses exceeding $3,100.

This radical change might prompt you to reconsider the size of your present auto and homeowner's coverage. You might want a larger policy, now that the government is no longer subsidizing your uninsured losses.

* Unemployment compensation. If you have been out of work and collecting unemployment benefits, your 1982 tax bill may be larger than you expected. Congress backdated a tax increase on excess unemployment pay to cover all of 1982.

Effective last Jan. 1, a single taxpayer will owe income taxes on unemployment compensation if his or her total income tops $12,000 (down from $20,000 under previous law). A married person filing jointly will be taxed if total income exceeds $18,000 (down from $25,000).

* Excise taxes. Starting next year, the tax on local and long-distance telephone calls rises to 3 percent, from one percent today. The tax on cigarettes doubles to as much as 16 cents on regular sizes and 33.6 cents on king sizes. Starting this month, airline ticket taxes went to 8 percent from 5 percent. Each person flying abroad will be hit for an additional $3.

* Expanded alternative minimum tax. Any tax whose name contains three multisyllabic adjectives deserves no space in a family newspaper. It's enough to say that the EAMT raises taxes on people who minimize their taxes by making maximum use of the many deductions permitted by law. In effect, they're punished for using legal means to wipe out large amounts of taxes. Part of their punishment is having to listen to their accountants' explanations of exactly how the expanded alternative minimum tax is going to work.