Hearing loss due to damage to the cilia — for example, from repeated exposure to loud noise or changes in the inner ear as we age — is largely untreatable because the cells cannot repair themselves. Hearing loss is also linked with tinnitus (constant ringing or buzzing noise in the ears) and with an increased risk of dementia, possibly because the brain has to direct more energy towards understanding speech.

Hearing aids can help by amplifying sound, but the hope is that the new drug could do away with the need for them by restoring healthy hearing. Developed by Frequency Therapeutics, a company linked to the Massachusetts Institute of Technology in the U.S., the gel contains a mixture of drugs that, in laboratory tests, helped new hair cells form from a type of stem cell called progenitor cells. One of the drugs included is valproic acid, a readily available and relatively cheap drug already used as an anti-convulsant for epilepsy. Unlike other types of stem cell, progenitor cells cannot develop into any form of body tissue. Instead they are more likely to develop into cells near where they are found in the body.