The GermBullet is sold online through retailers like CVS.com and at a handful of small pharmacies and natural food stores in Florida.
A man reached by phone at Flu and Cold Defense's office could not immediately comment on the warning letter.
This year's flu season started about a month earlier than normal and the dominant flu strain is one that tends to make people sicker. The government doesn't keep a running tally of adult deaths from the flu, but estimates that it kills about 24,000 people most years. Vaccinations are recommended for anyone 6 months or older.
Flu and Cold Defense issued a news release early in the month saying GermBullet "may help protect you so your immune system is not overwhelmed as the flu reaches epidemic levels." FDA and FTC regulators take issue with that statement and a number of others from the company's website, including that the inhaler is "shown to reduce illness-causing bacteria, cold and flu viruses and fungi."
The letter, dated Jan. 24, was posted to the FDA's website Tuesday.
Regulators gave the company 15 business days to correct the problematic statements.
"The FTC strongly urges you to review all claims for your products and ensure that those claims are supported by competent and reliable scientific evidence," states the letter.
The FDA regularly issues warning letters to companies that do not follow regulations for manufacturing and promoting drugs and medical devices. The letters are not legally binding, but the FDA can take companies to court if they are ignored.