Bipolar disorders affect approximately 4.5 million people across the European Union (EU) and are associated with high annual healthcare and societal costs. Bipolar disorder I and II represent disorders that cause extreme fluctuation in a person's mood, energy, and ability to function, in which symptoms of (hypo)mania and depression alternate. The depressive episodes of bipolar disorders are often referred to as bipolar depression (BD). In other words: it is a phase/state of the disorder. For many patients with BD, the depressive polarity is often more pervasive and more debilitating than manic states, with estimates that depressed mood accounts for up to two-thirds of the time spent unwell, even with treatment. The burden of not received an effective treatment for BD is high: more severe psychopathology, higher rates of unemployment, more hospitalisations, lower quality of life, lower cognitive functioning, risk of suicide, comorbidities and poorer social and occupational functioning and thus more carer burden. For BD, the treatment guidelines are very heterogeneous, amongst other reasons because the disease is heterogeneous and treatments should be tailored to the patients. There is no clear treatment algorithm and it cannot yet be predicted which treatment will be effective. Especially the place of adjunctive antidepressants is under debate. Usually, for psychiatric disorders (including bipolar disorder), a patient is considered to be treatment-resistant is two medicinal treatments have been tried (in sufficient duration and dosage) without sufficient success. For BD, there is no consensus on when to consider a patient as treatment-resistant, but the most common definition is after one prior treatment failure. This raises the research question whether adjunctive antidepressants to treat BD should be introduced earlier in the treatment. Additionally, The INTENSIFY trial is part of the larger Horizon 2021 project, with the central goal of paving the way for a shift towards a treatment decision-making process tailored for the individual at risk for treatment resistance. To that end, we aim to establish evidence-based criteria to make decisions of early intense treatment in individuals at risk for treatment resistance across the major psychiatric disorders of schizophrenia, bipolar disorder and major depression.