WASHINGTON — It’s one part of the new health-care law that seemed clear: free coverage for preventive care under most insurance plans. It didn’t turn out that way.

So Wednesday, the Obama administration tried to straighten out the confusion.

Some people have had a colonoscopy, thinking the procedure was free, only to get a hefty bill because the doctor removed a polyp. No more. Removing such precancerous growths as part of a routine colon-cancer screening will now be considered preventive care. The charge for a colonoscopy can be more than $1,000.

“Polyp removal is an integral part of a colonoscopy,” the Department of Health and Human Services said in guidance posted on its website.

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President Obama’s health-care law required most private health plans to cover preventive care at no additional charge to patients. For workers and their families, the expense is borne by the company health plan, which passes on some of those costs in the form of higher premiums.

Also addressed in the notice was genetic testing for breast cancer, coverage of over-the-counter products such as aspirin for heart care, nicotine patches for smoking, and birth control for women. Unlike formal regulations, the guidance does not have the force of law, but advocates for patients say insurers would be ill-advised to ignore it.

Other free preventive services addressed in Wednesday’s guidance:

• Insurers must cover testing, if ordered by a doctor, for rare BRCA genes that substantially increase the risk of breast cancer. The charge for such tests can be as much as $3,000.

• Over-the-counter products such as aspirin for heart care and nicotine patches for smoking cessation are covered with a doctor’s prescription.

• Insurers won’t be able to fulfill the law’s requirement to cover contraception as preventive care for women if they only pay for birth-control pills. A full range of FDA-approved methods must be covered, including long-acting implant and intrauterine devices. Birth-control methods for men are not covered as preventive care.

The insurance industry said the requirement to cover over-the-counter medications will lead to higher costs.