For a Botox injection, patients waited typically for eight days, while those asking doctors to look at worrisome moles that might indicate skin cancer waited 26 days, said the study in the Journal of the American Academy of Dermatology.
The study was conducted last year by telephone with researchers posing as patients, contacting 898 dermatologists in 12 US cities.
In Boston, for example, the median wait for a Botox session was 13 days while it was nine weeks for an examination of a changing mole.
In Seattle, an injection of botulinum toxin can be arranged in less than eight days but patients had a median wait of 35 days to secure an exam for a mole.
The lead author of the study, Jack Resneck, assistant professor of dermatology at the University of California, San Francisco, wrote that it was unclear why the wait times varied so dramatically.
It was possible doctors wanted to offer faster access for Botox treatments "because of higher relative payments for cosmetic services," the study said.