At least 18 people were taken to hospital late Tuesday suffering from severe stomach ache and vomiting.
“Thirteen people have died so far. Two more are critical and the rest have been discharged,” Rahul Kumar, magistrate of Gopalganj district, told AFP.
Kumar said tests were underway to determine the exact cause of death, but they are suspected to have all died after drinking moonshine from illegal bootleggers.
One of India’s poorest states, Bihar announced prohibition in April after chief minister Nitish Kumar promised at elections to crackdown on a widespread problem of alcohol abuse.
Bihar is one of a string of states to have banned alcohol over the years.
Hundreds of poor people who cannot afford branded liquor die every year in India from drinking cheap and toxic hooch.
Last month, 33 people died in neighbouring Uttar Pradesh state after drinking toxic brew.
Bootleggers often add methanol — a highly toxic form of alcohol sometimes used as an anti-freeze or fuel — to their home-brew liquor to increase the alcoholic content.
If ingested, it can cause blindness and liver damage and can kill in larger concentrations.
More than 100 people died in Mumbai last year after drinking illegal moonshine in a slum.
Nearly three billion litres of legal liquor and an estimated two billion litres of hooch are consumed in India annually, according to the International Spirits and Wines Association.