In Africa, use of medicinal plants for the treatment of diabetes is very common. However, efficacy on co-administering of medicinal plants with therapeutic drugs hasn't been fully determined, especially for African medicinal plants. The current study focused on assessing the in vitro modulation effects of three popular African medicinal plants, namely: Aloe ferox, Sutherlandia frutescens and Prunus africana (including five commercial preparations containing these medicinal plants) on two of the most important anti-diabetic drug metabolising enzymes, Cytochrome P450 (CYP450) 2C9 and CYP3A4 and a key drug efflux transporter, P-glycoprotein (P-gp). Vivid® microsome-based screening kits were used to assess inhibitory potency of plants preparations on CYP2C9 and CYP3A4 enzymes activities. The study showed that P. africana was a more potent inhibitor of CYP2C9 and CYP3A4 activity than the corresponding positive controls Ginkgo biloba and St. John's wort, which are known to cause clinically significant drug-herb interactions. S. frutescens leaf extract demonstrated potent to moderate inhibition on both the tested CYP activities, while its commercial products (Promune® and Probetix®) possessed moderate to mild inhibitory effects on the activities of both CYPs. Potent inhibitory effect on CYP2C9 and CYP3A4 was seen with Aloe Ferox®. Prosit® and Aloes powder® showed potent to moderate inhibition on CYP2C9 activity and moderate to mild inhibition on CYP3A4 activity. In addition to CYP450 activity, the present study also investigated the effects of the selected medicinal plant products on the activity of the main drug efflux protein, P-gp. A screening assay was specifically developed to assess the potential for herbal remedies to interact with P-gp mediated drug absorption. The assay is based on the principle of the reversal of drug resistance in modified Caco-2 cells specifically altered to express high iv efflux protein activity. These cells display a multidrug resistance phenotype and the addition of a plant extract containing a P-gp inhibitor or substrate will inhibit or compete with any cytotoxic drug and consequently reverse the drug resistance. The suitability of the assay was confirmed using a known P-gp inhibitor. The study observed that the anti-proliferation effect of vinblastine was significantly enhanced in vinblastine-resistant Caco-2 cells, which have high P-gp expression, when they were exposed to the selected African herbal preparations. This observation indicates that the studied plant preparations may alter P-gp functionality and therefore lead to interference with the absorption of co-administered drugs. The outcomes of this study provide useful information on whether there are any potential drug-herb interactions between the commonly used African medicinal plants and oral anti-diabetic drugs, at the level of CYP and P-gp drug metabolism and could contribute to better therapeutic management of Type II diabetics. However these predicted interactions will need to be verified in a clinical setting.