For months, as President Trump threatened to impose punishing tariffs on imported medicines, fears mounted that American patients would be harmed by higher prices and shortages of vital drugs.

The details of the drug tariffs Mr. Trump announced on Thursday night are still coming into focus. Here’s what is known so far.

Will my medicine be subject to the new tariffs?

For the vast majority of prescription drugs taken in the United States, the answer is no.

Cheaper, old generic drugs, which account for about 90 percent of Americans’ prescriptions, will face no tariffs, a Trump administration official confirmed on Friday.

What if I’m taking a brand-name drug?

If your brand-name drug is made in the European Union, as many are, it could soon be subject to a tariff of up to 15 percent. It is not clear yet when that will go into effect. Some of the best-known and most widely taken brand-name drugs are made in the bloc.

If your medicine is made in the United States, the second-most common region for brand-name drug manufacturing, it should not be affected by the tariffs.

If your drug is made in certain other countries, it could possibly soon be hit by the tariff of 100 percent. Some of the drug manufacturing that could potentially be vulnerable to that is in Switzerland, Britain, Singapore, India, China, Canada and Mexico. But those countries make a relatively small share of Americans’ brand-name drugs.