Drug interaction is a major cause of drug attrition from the market. Cytochrome P450 (CYP) and glutathione transferases (GST) are superfamilies of important drug-metabolizing enzymes. Modulation of these enzymes by drugs could have adverse consequences. Concomitant intake of two or more medicines could result in drug-drug, herb-drug or herb-herb interactions that could result in therapeutic failure or toxicity among others. Therefore, it is crucial that during the preclinical stages, drug candidates are screened for their potential to alter the metabolism of drugs and impact clinical outcomes. The Department has built the capacity over the years to investigate the drug interaction potential of drug candidates in vivo and in vitro using animal models and recombinant drug-metabolizing enzymes, respectively. Plant medicines from some major Herbal Centres in Ghana that are used to treat/manage tuberculosis, malaria and HIV/AIDS have also been evaluated for their potential to cause CYP-mediated drug interactions. Glutathione transferases play a key role in the body’s defense system, mopping up free radicals and xenobiotics that are potentially harmful. Thus, we investigate the effect of the drug candidates for their effect on GSTs. CYP and GST inhibitory potential have been documented for the plant medicines used to manage tuberculosis, malaria and HIV/AIDS, and other medicinal plants.