On the heels of President Donald Trump’s Monday announcement of an executive order that will slash prescription drug prices in the U.S., the spotlight is on current costs and how much Americans could save.

The president’s order calls for "most favored nations drug pricing" — which means "the lowest price paid for a drug in other developed countries, that is the price that Americans will pay," he said.

"Some prescription drug and pharmaceutical prices will be reduced almost immediately by 50 to 80 to 90%," Trump said. 

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Katy Dubinsky, a New York pharmacist and founder and CEO of Vitalize, applauded the move to reduce prescription prices, noting that Trump’s order tackles a long-standing problem.

"But this will not be simple to accomplish," she told Fox News Digital. 

"The executive order doesn't reduce costs immediately," she said. "It directs government agencies to start drafting the rules, which may take months."

Here are the five most expensive prescription drugs in the U.S. by price — followed by five by volume.

Dubinsky detailed some of the most expensive prescription drugs in the country today and what conditions they treat.

1.  Lenmeldy (atidarsagene autotemcel) by Orchard Therapeutics – $4.25 million

This medication is used to treat metachromatic leukodystrophy (MLD), a rare genetic disorder that damages the nervous system, Dubinsky said. 

"It is given once and is supposed to stop or slow down the disease in young kids," she noted.

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2. Hemgenix (etranacogene dezaparvovec-drlb) by CSL Behring – $3.5 million

This medication is prescribed for people with hemophilia B, a bleeding disorder. 

"This one-time treatment helps the body make its own clotting factor, so patients don’t need regular infusions," said Dubinsky.

3. Elevidys (delandistrogene moxeparvovec-rokl) by Sarepta Therapeutics – $3.2 million

This prescription medication, intended for young boys, treats Duchenne muscular dystrophy (DMD), a condition that weakens muscles over time. 

"It aims to slow down how fast the disease progresses," Dubinsky said. 

4. Skysona (elivaldogene autotemcel) by Bluebird Bio – $3 million

"This medication is used for cerebral adrenoleukodystrophy (CALD), a serious brain disease in boys," said Dubinsky. "This therapy tries to slow the damage before symptoms get worse."

5. Zynteglo (betibeglogene autotemcel) by Bluebird Bio – $2.8 million

Zynteglo is for beta-thalassemia, a blood condition that usually requires regular transfusions. 

"This gene therapy can help patients make healthy red blood cells on their own and reduce how often they need treatment," said Dubinsky.

John Stanford, executive director of Incubate, a Washington-based coalition of early-stage life-science investors, shared his thoughts on the top five most expensive drugs by sales volume.

"Typically, when the government is focused on the most expensive drugs, they're focused on the metric based on sales volume rather than, for instance, a rare disease therapy with a high list price but smaller patient pool," he told Fox News Digital.

"Often, officials are focused on total drug spending by Medicare or other government programs."

1. Keytruda (pembrolizumab) by Merck — $25 billion revenue (2023)

Keytruda is an immunotherapy medication used to treat a variety of cancers, including melanoma, non-small cell lung cancer, liver cancer and others.

"Keytruda has become Merck's crown jewel, helping the company expand its cancer treatment portfolio with more than 1,000 active clinical trials," Stanford told Fox News Digital.

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2. Eliquis (apixaban) by Bristol Myers Squibb and Pfizer — $18.95 billion

Eliquis (apixaban) is an "anchor drug" for both BMS and Pfizer, according to Stanford.

Apixaban is prescribed to prevent the formation of blood clots and to treat deep vein thrombosis and pulmonary embolism (a blood clot in the lungs). 

3. Ozempic (semaglutide) by Novo Nordisk — $13.93 billion

Prescribed for type 2 diabetes, the semaglutide medication Ozempic has become widely popular for its weight-loss effects and other health benefits.

"Ozempic's sales are powering Novo Nordisk's broader foray into GLP-1s for obesity, heart disease and liver conditions — all areas with high development costs and uncertain scientific outcomes," Stanford told Fox News Digital. 

"The money has gone toward scaling up production to meet demand for GLP-1s and avoid supply shortages."

4. Humira (AbbVie) — $14.4 billion (U.S. 2023 revenue)

"Humira has been one of the highest-grossing drugs in history, generating over $200 billion during its exclusivity period," Stanford said.

The injectable medication, which contains the active ingredient adalimumab, is used to treat rheumatoid arthritis and other inflammatory conditions.

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5. Biktarvy by Gilead — $11.85 billion

Biktarvy is an HIV treatment that includes the three ingredients bictegravir, emtricitabine and tenofovir alafenamide.

"Biktarvy isn't just a leading HIV treatment — it's the financial backbone for Gilead's move into cancer research," Stanford said.

Dr. Jacob Glanville, CEO of Centivax, a San Francisco biotechnology company, said vaccines and most generic drugs would not likely be changed by the executive order.

"Most vaccines that Americans take cost less than a hundred dollars, while generic drugs are often less than a dollar a pill," he told Fox News Digital.

What would be affected, Glanville predicted, are newer brand-name drugs still under IP exclusivity, antibody therapies, cellular therapies, gene therapies and personalized cancer vaccines.

"Some of these are excruciatingly expensive — $100,000 to $500,000 for a treatment course for a patient. However, they are also often the most effective treatments for certain cancers, autoimmune disorders or rare diseases."

The pharmaceutical industry might argue that lowering the prices on these medicines will result in a "dramatic reduction of investment" in creating such breakthroughs, said Glanville.

The industry may also argue that these medicines eventually become generic — at which point the prices drop, according to the expert.

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"From a patient’s perspective, the price of medical care in the United States is unsustainable, and extremely expensive medicine is part of that," he said. But "the insurance system and the hospital business also contribute."

"If the prices of new medicines are capped, then effort should be made to reduce the cost of clinical trials and drug GMP manufacturing. Otherwise, we will lose a lot of innovation."

Greg Norman of Fox News Digital contributed reporting.