Several causal observations and a few recent studies suggest that left-handers may have greater reactivity to various drugs. A sample of 747 adults was surveyed to determine if they had had signs of hypersensitivity to commonly prescribed pharmaceutical drugs. Left-handers were significantly more likely to experience constipation, dizziness, and skin rashes, and were nearly three times more likely to experience other, miscellaneous, negative drug reactions. In addition left-handers were nearly twice as likely to have experienced situations where their physician felt it necessary to reduce drug dosages because of unwanted side-effects of medication. A second experiment using 840 adults indicated that a potential confound that would result if left-handers simply used more medical drugs in general, which would then give them more occasions on which reactions could occur, does not explain these results. Mechanisms that might account for these differences in drug sensitivity include differences in brain morphology, birth stress related neuropathy, and differences in immune system responses, all of which have been found to differ as a function of handedness.