Regular use of non-steroidal anti-inflammatory drugs (NSAIDs) may be protective against tumours, including breast cancer. We have studied the effects of ibuprofen and aspirin on DNA damage in lymphocytes obtained from breast cancer patients and healthy female controls. Both nanoparticle (NPs) and bulk formulations were used in the comet and micronucleus (MN) assays. Non-toxic doses (250 ng/ml ibuprofen; 500 ng/ml aspirin) were tested. Aspirin, both bulk and nano formulations, significantly reduced DNA damage measured with the comet and micronucleus assays; the nano formulation was more effective. Ibuprofen was not effective in the comet assay but showed a significant reduction in MN frequency, with the nano formulation being more effective. NPs may have better penetration through the nuclear membrane relative to the bulk formulation. NSAIDs such as aspirin and ibuprofen may have a promising role in cancer prevention and treatment.