Abstract Background Side effects of cancer treatment have a high impact on patients. Besides physicians and nurses, other patients are an important partner for communication on side effects. Online fora within self-help groups offer a digital option for this communication. Our aim was to assess the content of the largest German self-help forum for cancer patients with respect to side effects of cancer treatment. Methods All threads in the self-help forum of the German women’s self-help (11/17/2012 to 06/01/2021) under the topic “side effect management”- “how can I help myself?” were assessed quantitatively (topic, duration, number of answers at all and per day at peak) and regarding their content. Results The vast majority of patients mentioned different kinds of pain as side effects, followed by infection or necrosis. While most threads showed a target-oriented search to obtain specific information, a quarter also dealt with strong emotions (anxiety, worry, despair). The main goal in starting a thread was to find information and other patients with similar problems. Most reactions (visits and answers) were received by threads on side effects of chemotherapy. Conclusions The forum of a self-help group allows interactive patient-centred discourse between members independent of place and time. It provides insight into the meaning and burden of side effects from the patients’ perspective. This may help to optimise the support of cancer patients.