In this article, we analyse how health professionals educate cancer patients to care for their condition and keep strict control over therapy safety. We study how much room for negotiation is left to patients during medical consultations so resources can still be exchanged. We pay particular attention to the trade of knowledge and powers between patients and doctors (power to act and to express oneself in an imbalanced relationship where knowledge is unequally shared). We opted for a qualitative approach with 41 interviews and several ethnological observations, first of consultations in haematology, then of pre-planned phone calls made to patients during the course of a cancer therapy follow-up scheme. The declared ambition of turning cancer patients into self-responsible patients actually re-enacts well-known procedures of control and knowledge acquisition aimed at narrowing their margin of manoeuvre for the sake of therapy safety. Even if some freedom is conceded, patients remain under the control of their medical hierarchy. Health professionals privilege two methods to keep control over patients and teach them therapy safety procedures. Which method is chosen, and how it is used, is dictated by the relationship between socially-diverse patients and health professionals. In the end, what the patient learns and the amount of control the doctor keeps over this process will depend on the distribution of power and knowledge among them, but asymmetry will always remain.