During the first three months of the section 3 you can be treated and medicated against your will, even with injections and antipsychotics and in some cases even electroconvulsive therapy (ECT)- (in emergencies or if the treatment is authorised by the mental health act comissioners panel). After three months, you must consent to treatment. If you do not, an independent doctor must confirm that the patient can still be treated against their will if it remains in their best interests.
from
the mental health information page at the top of the page.