Sharing in-earphones is even worse, they discovered, as bacteria can get passed from one user to another.
The Indian scientists discovered that regular users of iPods and other mp3 players had higher rates of bacterial infection than those who were not regular users.
The small study at Kasturba Medical College in Manipal, south west India, took 50 health medical students, half who said they were regular users (Group B) and half who said they were not (Group A).
The researchers found: "Group B showed heavy growth and a significant increase in the number of bacterial growths after frequent and constant use."
Bacteria was found in the ears in 92 per cent of those who did use in-earphones regularly and 80 per cent of people who did not.
They concluded: "Frequent and constant use of earphones increases the bacterial growth in the ear."
Sharing earphones "might be a potential vector" for bacteria, they added.
Bacteria they discovered include types of Staphylococcus, that can cause swelling in the ear and earache.
While Staphylococcus is a normal skin flora, it can cause problems if it manages to break the surface, such as with a minor abrasion or wound. Skin in the outer ear is particularly delicate.
The microbiologists said: "The majority of the isolates were Staphylococcus spp. (MSCONS) which although a normal skin flora most often, could well be a pathogen if the condition is congenial. Cheaper quality [of earphones] or improper use may give rise to abrasion, leading to breakage of the skin which might be a portal of infection."
They added those in hospital should be cautious about using in-earphones because of their potential to introduce drug-resistance strains of Staphylococcus into the body, such as MRSA.
They said: "It might be speculated that if the earphones are also used in the hospitals during leisure hours, it might give rise to colonization of potential pathogens that may be drug resistant on some occasions."
If people did share in-earphones, they concluded, they must be careful to clean them beforehand, preferably with alcohol.
They published their findings in the Online Journal of Health and Allied Sciences.