ObjectiveThe synergism among extracts ofSenna alata, Ricinus communis,andLannea barteri,and their anti-infective activities were investigated. The data collected for the antimicrobial activity of the extracts combinations were interpreted to be one of the following categories: synergy; indifferent; additive; or antagonistic. The interpretation was made based on the fractional inhibitory concentration index (FICI) results. FICI of ≤ 0.5 indicates synergism, > 0.5 to 1 indicates additive effects, > 1 to ≤ 4 indifference, and > 4 is considered to be antagonism.ResultsCompared with the data of the individual extracts, the MIC values of the extract-extract combinations against all strains of the tested microorganisms were significantly lower, ranging from 0.97 to 1.17, 0.97 to 4.69, 0.50 to 1.17, 1.17 to 3.12 and 2.34 to 4.69 mg/mL forEscherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, Klebsiella pneumonia and Candida albicansrespectively.L. bateriaqueous-S. alataethanol extracts andS. alataaqueous-R. cummunisethanol extracts combinations showed a synergy effect against all the test microorganisms. The other combinations exhibited at least one additive effect. Neither antagonism nor indifference activity was observed. This study validates the relevance of combining these plants in treating infections by traditional medicine practitioners.Supplementary InformationThe online version contains supplementary material available at 10.1186/s13104-023-06354-7.