This article focuses on the drug–drug interaction between acetaminophen (also known as paracetamol) and β-lactam antibiotics (amoxicillin, ampicillin and penicillin) studied by means of cyclic voltammetry and controlled potential coulometry. The results indicate a drug–drug interaction between N-acetyl- p-benzoquinone-imine (NAPQI) derived from the anodic oxidation of acetaminophen and β-lactam antibiotics. Also the cyclic voltammetric data were analysed by digital simulation to measure the observed homogeneous rate constants ( k obs ) of the reaction of NAPQI with β-lactam antibiotics. The calculated k obs for the reaction between NAPQI and β-lactam antibiotics was found to vary in the order k obs amoxicillin = k obs ampicillin > k obs penicillin at biological pH.