Asthma sufferers gather every June in the southern city of Hyderabad to gulp down the fish stuffed with a yellow herbal paste, in hopes it will help them breathe more easily.
The wriggling five-centimetre (two-inch) fish are slipped into the throats of patients in a bizarre treatment that leaves them gagging.
The Bathini Goud family, which administers the treatment, says the fish clear the throat on their way down and permanently cure asthma and other respiratory problems.
But the family has declined to reveal the secret formula which they say they got from a Hindu saint in 1845.
Parents are often forced to pry open the mouths of reluctant children who cry at the site of squirming fish, while others pinch their noses, tip their heads back and close their eyes.
Thousands of people travel from across India for the free medicine during a two-day period, the specific dates of which are determined by the onset of the monsoon every June.
Rights groups and doctors have complained that the medicine is "unscientific", a violation of human rights and unhygienic, claims rejected by the family.
The Indian government arranges special trains for the "fish medicine" festival every year and extra police are on duty to control crowds.
After digesting the treatment, patients are told to go on a strict diet for 45 days.