WASHINGTON — When President Trump unveiled his plan to lower prescription drug prices in a Rose Garden speech last month, he said he would inject more competition into the market by bolstering negotiating powers under Medicare. But experts analyzing the plan warn of a possible side effect: The proposal could significantly increase out-of-pocket costs for some of the sickest people on Medicare.

At the heart of the president’s plan is a proposal to switch some expensive drugs from one part of Medicare to another part — moving them from Part B, the medical benefit created in the original 1965 Medicare law, to Part D, the outpatient drug benefit added by Congress in 2003.

Under Part D, the government contracts with private health insurance companies to manage the benefit and negotiate discounts with drugmakers. There is no such negotiation for the drugs covered by Part B, which are administered by infusion or injection in doctors’ offices or hospital outpatient departments.

But Medicare beneficiaries typically pay a larger share of the costs for Part D drugs. Many beneficiaries have supplemental insurance, such as a Medigap policy, to help pay their share of the bill for drugs covered under Part B. Medigap policies are not allowed to cover Part D expenses.