The prevailing opinion is that prefibrillar β-amyloid (Aβ) species, rather than end-stage amyloid fibrils, cause neuronal dysfunction in Alzheimer's disease, although the mechanisms behind Aβ neurotoxicity remain to be elucidated. Luminescent conjugated oligothiophenes (LCOs) exhibit spectral properties upon binding to amyloid proteins and have previously been reported to change the toxicity of Aβ1-42 and prion protein. In a previous study, we showed that an LCO, pentamer formyl thiophene acetic acid (p-FTAA), changed the toxicity of Aβ1-42. Here we investigated whether an LCO, heptamer formyl thiophene acetic acid (h-FTAA), could change the toxicity of Aβ1-42 by comparing its behavior with that of p-FTAA. Moreover, we investigated the effects on toxicity when Aβ with the Arctic mutation (AβArc) was aggregated with both LCOs. Cell viability assays on SH-SY5Y neuroblastoma cells demonstrated that h-FTAA has a stronger impact on Aβ1-42 toxicity than does p-FTAA. Interestingly, h-FTAA, but not p-FTAA, rescued the AβArc-mediated toxicity. Aggregation kinetics and binding assay experiments with Aβ1-42 and AβArc when aggregated with both LCOs showed that h-FTAA and p-FTAA either interact with different species or affect the aggregation in different ways. In conclusion, h-FTAA protects against Aβ1-42 and AβArc toxicity, thus showing h-FTAA to be a useful tool for improving our understanding of the process of Aβ aggregation linked to cytotoxicity.