Mr Trump was hospitalised at the Walter Reed Medical Centre on Friday evening as a precaution, after he showed symptoms of coronavirus following a positive Covid-19 test result a few hours earlier.
Over the weekend, the medical team at the hospital administered the president with dexamethasone, as part of his treatment for Covid-19.
Dexamethasone is a corticosteroid that has been used during the pandemic to help treat a limited number of coronavirus patients. One study suggested that it reduced the risk of death among ventilated Covid-19 patients by around 33 per cent, according to Newsweek.
The treatment, which has not been approved by the Food and Drug Administration (FDA) for widespread use, has been recommended by the National Institutes of Health (NIH) for hospitalised coronavirus patients, who are being treated with ventilators or are requiring oxygen.
Speaking to MSNBC host Andrea Micthell on Thursday, Dr Fauci, one of the country's top infectious disease experts and part of the US government's coronavirus task force, said the treatment can make a patient “very energetic”.
He told Mitchell: “When you're on steroids, there are a number of side effects. It makes you hyper energetic, it sometimes interferes with your ability to sleep.
“If you're on it [dexamethasone] a very long time, and which he's not, and they don't plan on having him on it for a long period of time, there are long range side effects.”
Dr Fauci added: “But the side effects when a person is on a higher dose for a short time, it's more that you're very energetic.”
The NIH website lists several other side effects of the drug, including psychiatric effects, infections and dangerous levels of sugar in the patient's blood.
Following the announcement that the president had been treated with dexamethasone last weekend, several health experts raised concerns about the effects of the steroid.
Dr Ashish Jha, dean of Brown University's School of Public Health, told Fox News at the weekend: “The president is going to get a short course so I’m not worried about long-term effects of dexamethasone.
“But we definitely see in 30-40 percent of people pretty substantial effects...[of] the anxiety, the agitation.”
While, Dr Edward Jones-Lopez, an infectious disease specialist at the University of Southern California, told Reuters that “Steroids are always very dangerous medications to use.
“That is why it [dexamethasone] is used in severe to critical patients...There can be neuropsychiatric side effects. These are medications that we use very, very carefully.”
Michele Dauber, a law professor at Stanford University who was previously treated with dexamethasone, claimed on Sunday evening that Mr Trump would be unable to lead the country after taking the drug.
“I was treated with dexamethasone following brain surgery. It is (as my team told me) a drug that seriously messes with your mind. It is a bad drug. I could not wait to get off it.
“Unfortunately you have to wean off which takes time. Trump is incapacitated,” she tweeted on Sunday.
Ms Dauber added in a follow-up tweet: “I couldn't be president of my cat when I was on dexamethasone.
“He should not be exercising the powers of the office of president on that drug. We are lucky if he doesn't start a war. He's incapacitated.”
Concerns were raised about the president’s reaction to the treatment last week, when he left the medical centre on Sunday to wave to his supporters who had waited for him since his admission to the hospital.
President Trump had only been showing symptoms of Covid-19 for three days at this point, and the two members of his security detail who were in the car with him have subsequently been forced to quarantine for 14 days following the brief trip.
On Monday, Dr Fauci refused to comment on president Donald Trump’s decision to leave the Walter Reed Medical Centre briefly, while speaking to CNN host John Berman.
The host asked Dr Fauci during the interview if it was safe for Mr Trump to leave the medical centre for the short trip.
Dr Fauci replied: “I don't want to really go there, John, and comment on that, OK?”
According to a tracking project hosted by Johns Hopkins University, there are now more than 7.5 million people who have tested positive for coronavirus in the US. The death toll has reached at least 212,514.