Health care legislation drafted by the House of Representatives would boost senior citizens' prescription drug-coverage premiums an average of 20 percent by 2019, the nonpartisan Congressional Budget Office said Friday.

However, seniors' overall spending on drugs would decrease, according to the CBO report.

The CBO said the premium costs would increase partly because the House legislation is designed to end an existing gap in Medicare prescription drug coverage known as the "doughnut hole."

Currently, about a quarter of Medicare patients with prescription drug coverage fall into the doughnut hole coverage gap. The gap in coverage occurs once the cost of a patient's prescriptions exceeds about $2,700. Patients have to cover the next $4,350 on their own until Medicare coverage kicks in again. The House bill would close the gap gradually until it was eliminated in 2022. Before then, beneficiaries would get a 50 percent discount on many brand-name drugs in the doughnut hole under the House bill.