Citing chronic drug shortages, however, the Federal Aviation Administration has granted airlines exemptions that permit passenger planes to fly without a complete medical kit if the airlines say they cannot replenish the drugs. The exemptions apply to international as well as domestic flights.

Earlier exemptions were issued by the agency on an annual basis, and only for one or two of the drugs in the kit. But in January 2016, more than 50 airlines were granted four-year exemptions from the requirement to carry all five drugs in the medical kit.

The medicines include two doses of epinephrine, one to treat severe allergic reactions and one to treat cardiac arrest; atropine, which is used to treat a slow heart rate; dextrose, to raise dangerously low blood sugar in people with diabetes; and lidocaine, to treat irregular heart rhythms but rarely used these days.

There are no data on how many airplanes may be flying at a given time without the drugs. Representatives of airlines said they usually carry complete medical kits and rely on the exemption only during periods of temporary shortages.

“U.S. carriers adhere to all F.A.A. regulations, including carrying emergency medical kits on board the aircraft and providing flight crews with training for handling medical emergencies,” said Katherine Estep, a spokeswoman for Airlines for America, an industry group that secured exemptions for all of its members, including American Airlines, JetBlue, Southwest and United Airlines.