Due to the increase in new types of cancer cells and resistance to drugs, conventional cancer treatments are sometimes insufficient. Therefore, an alternative is to apply nanotechnology to biomedical areas, minimizing side effects and drug resistance and improving treatment efficacy. This work aims to find a promising cancer treatment in the human colorectal adenocarcinoma cell line (HT-29) to minimize the viability of cells (IC50) by using carbon nanotubes (CNTs) combined with different drugs (5-fluorouracil (5-FU) and two repurposing drugs—tacrine (TAC) and ethionamide (ETA). Several CNT samples with different functional groups (-O, -N, -S) and textural properties were prepared and characterized by elemental and thermogravimetry analysis, size distribution, and textural and temperature programmed desorption. The samples that interacted most with the drugs and contributed to improving HT-29 cell treatment were samples doped with nitrogen and sulfur groups (CNT-BM-N and CNT-H2SO4-BM) with IC501.98 and 2.50 µmol∙dm−3from 5-FU and 15.32 and 15.81 µmol∙dm−3from TAC. On the other hand, ETA had no activity, even combined with the CNTs. These results allow us to conclude that the activity was improved for both 5-FU and TAC when combined with CNTs.