Breast cancer is a leading cause of cancer deaths in women within the United States. However, current treatment methods for the disease present deleterious side effects themselves. Therefore, there is a move towards finding natural cures in order to mitigate negative side effects while still providing effective treatment for the cancer. Blackseed (Nigella sativa) oil is one particular natural remedy, alongside its active ingredient thymoquinone (TQ), which has been successfully tested for suppressing certain types of breast cancer cell proliferation. TQ itself has been seen to be capable of preventing proliferation of both non-aggressive MCF-7 and highly aggressive MDA-MB-231 cancer cells. However, studies which looked at the effects of TQ on MCF-7 cells alone were limiting in their use of high concentrations of the chemical without emphasis on finding a minimum effective dosage. Additionally, a second study which tested the effects of TQ on both MCF-7 and MDA-MB-231 cell lines conducted the experiments in the presence of a lipid-carrier molecule. This, in turn, may have served as a confounding variable in the results. Therefore, it was hypothesized that a minimal effective dosage for both blackseed oil and TQ could be determined, where a significantly greater suppression of MDA-MB-231, in comparison to MCF-7, cell proliferation would be observed. Cell proliferation, cell adhesion, and soft agar assays were used to test the hypothesis of this study. The minimum effective dosage for each substance, characterized by proliferation of the non-aggressive MCF-7 cells to some extent and suppression of the aggressive MDA-MB-231 cells, were determined to be 0.5 µL for blackseed oil and 1.0 µM for TQ. Additionally, TQ’s effectiveness was noted to be more time-dependent than blackseed oil. This study supports the use of minimal effective doses for blackseed oil or TQ to naturally treat breast cancer while preventing damage to non-aggressive cells. KEYWORDS: Breast cancer; Blackseed oil; Nigella sativa; Thymoquinone; Effective dose; Natural remedies