Medical experts are calling for global action to tackle the viruses that cause the liver disease hepatitis.
The first worldwide estimates in drug users show 10 million have hepatitis C while 1.3 million have hepatitis B.
Writing in the Lancet, experts say only a fraction of those who could benefit are receiving antiviral drugs.
Only one in five infants around the world are vaccinated against hepatitis B at birth, they say.
The figures, published in the Lancet, show about 67% of injecting drug users in the world have been exposed to hepatitis C, while around 10% have come into contact with hepatitis B.
In the UK, around half of injecting drug users have been infected with the hepatitis C virus, while the rate for exposure to hepatitis B is 9% - the highest in western Europe.
The research was led by Prof Louisa Degenhardt of the Centre for Population Health, Burnet Institute, Melbourne, Australia, and Paul Nelson from the National Drug and Alcohol Research Centre at the University of New South Wales.
They say: "The public-health response to blood-borne virus transmission in injecting drug users has mainly centred on HIV.
"Maintenance and strengthening of the response to HIV in injecting drug users remains crucial, but the significance of viral hepatitis needs to receive greater attention than it does at present."
Commenting on the study in the Lancet, Dr Joseph Amon, of Human Rights Watch, New York City, US, said: "This study provides us with a first step and powerful data to draw attention to the problem of viral hepatitis in people who use drugs.
"The next step is to challenge governments to act, and hold them accountable for implementation of rights-respecting and evidence-based programmes."
Hepatitis is caused by five main viruses - A, B and C, and, more rarely D and E.
Hepatitis B is the most common, and can be passed from mother to baby at birth or in early childhood as well as through contaminated injections, injected drug use, unprotected sex and other means.
Hepatitis C is also spread through using unsterile needles and less commonly through unsafe sex, and sharing razors or toothbrushes, for example.
The E virus, caught from infected water or food, is a common cause of outbreaks of the disease in developing countries, said the World Health Organization.
Many of those carrying hepatitis are not aware they have it and can unknowingly transmit it to others.