Hypersensitivity to interference (HYSTI) is a situation in which a person has a severe difficulty in memorizing verbal items that are similar to each other. This may result in induced dyscalculia: HYSTI was shown to correlate with a difficulty in learning the multiplication table, presumably because the multiplication table, which is memorized verbally, has much similarity between the items ("six times seven equals forty two", "six times eight equals forty eight", etc.). Here, we show causal evidence that HYSTI disrupts the memorization of multiplication facts. We report DL, a woman with HYSTI who had extremely poor knowledge of the multiplication table. To examine whether her multiplication difficulty resulted from HYSTI, we tested whether she could learn multiplication facts when interference was reduced. In a series of merely 12 short sessions over a period of 4 weeks, DL rehearsed 16 multiplication facts – four facts per week. When the 4 facts in a given week were similar to each other, DL’s learning was poor. Conversely, when the 4 facts in a given week were dissimilar from each other, DL learned them quickly and easily. The effect of similarity was observed during the training period and persisted at least two months after the end of training. These results provide the first causal evidence that HYSTI impairs the learning or retrieval of arithmetic facts. From a pedagogical perspective, our findings may call for re-considering how multiplication facts should be taught in elementary school.