Background Breast cancer is the most common type of cancer in women worldwide. The benefits of chemotherapy vary depending on the treatment regimen used and the characteristic of the tumour. However, adverse drug reactions associated with chemotherapeutic agents can cause dose delays or reductions; thereby, affecting the treatment outcomes. Objective To explore adverse drug reactions of chemotherapeutic agents used to treat breast cancer from the patients’ perspective. Methods A total of 110 threads from nine online discussion forums were evaluated. They were exported into NVivo for Mac where content analysis was applied. Threads were read carefully to observe emerging patterns which were then coded into sub-themes and grouped into main themes. Results The participants’ characteristics on online discussion forums were often missing. Four hundred and eleven participants experienced 473 adverse drug reactions that were mainly associated with the nervous and immune systems. The forums’ analysis yielded three main themes: patient–patient advice, self-medication and lifestyle changes. Conclusion Online discussion forums proposed valued source of data on adverse drug reactions associated with chemotherapeutic agents and overall patients’ experience with cancer. The adverse drug reactions experienced by patients changed their priorities and the way they dealt with the disease. Therefore, healthcare professionals must consider the patients’ experience and attitudes towards cancer when designing a treatment plan. This can be established by increasing communication between healthcare professionals and patients.