(CNN) Pilots are prohibited within two days of flying from experimenting with two yet unproven medications to prevent coronavirus infection, the Federal Aviation Administration has determined.

"Use of chloroquine or hydroxychloroquine to prevent coronavirus infection is disqualifying while on the medication and for 48 hours after the last dose before reporting for flight or other safety related duties," a new FAA directive says.

The text was shared with CNN by the Allied Pilots Association, which represents pilots at American Airlines. The directive was signed by Dr. Penny Giovanetti, the director of the Medical Specialties Division within the Office of Aerospace Medicine.

President Donald Trump has touted the medications, chloroquine and hydroxychloroquine, as potentially efficacious against the coronavirus.

But medical experts have warned the early indications are not firm evidence the medications can prevent infection and that there are "potential serious implications" to taking either preventatively.

The FAA said in a statement to CNN that it "takes a conservative approach" in reviewing medications and the ability of pilots "to do their jobs safely." The agency sets health standards for pilots and requires each possess a medical certification of physical fitness.

"Chloroquine and hydroxychloroquine were both reviewed by the FAA Federal Air Surgeon when they entered the market and have long been considered generally incompatible for those performing safety related aviation duties," the agency said.

FAA does allow limited exceptions for pilots with arthritis to take small doses of hydroxychloroquine. It says pilots who take the medication but meet a number of other criteria may take an eye examination and receive a special certification.

The FAA guidance said there is "no satisfactory scientific evidence that use of these medications decreases the severity of the virus" and noted "there is no standardized protocol," such as how much and how frequently to take the drug.

"Exercise of social distancing and hand washing represent a far more effective means of prevention," the guidance said. "As information changes, we will update our policy based on the best scientific evidence available to us."

Three US-based cardiovascular groups recently warned doctors should consider "potential serious implications for people with existing cardiovascular disease" if prescribing the treatment.

Prior to joining the FAA in 2012, Giovanetti served for 27 years in the Air Force, according to an FAA bio. Her credentials include special diplomas from the American Board of Preventive Medicine.