Scientists have discovered the drugs are being made available by 45 per cent of online pharmacy websites.
Imperial College London researchers surveyed 20 pharmacies accessed online by people in the UK.
The illegal supply of the medicine could be contributing to antibiotic resistance.
Published in the Journal of Antimicrobial Chemotherapy, the research found 80 per cent of online pharmacies allowed their customers to choose their own dosage.
Online pharmacies providing products to people in the UK are required to register with Medicines and Healthcare products Regulatory Agency (MHRA) and the General Pharmaceutical Council.
But three quarters of those surveyed didn’t show evidence of this registration.
Less than a third (30 per cent) made customers fill out a health questionnaire before they purchased the drugs.
Safe usage and side effect information was only provided by 70 per cent of the providers.
Professor Alison Holmes from Imperial College’s Department of Medicine said: “Improper use of antibiotics can mean that infections are not being treated appropriately, or that people are being unnecessarily exposed to antibiotics.
“This allows bacteria to become resistant to drugs that once killed them.
“As a result, it is essential that antibiotics are prescribed only when they are needed.”
Researchers reported all illegal providers to the Medicines and Healthcare products Regulatory Agency (MHRA), who are now working to improve safety for patients.