There are different types of psychological treatments for a wide variety of health problems, including depression. These treatments vary according to the theoretical foundation on which they are based, the level of formal development of their techniques, the available studies that allow endorsing their efficacy and their use by health professionals of the health system.

The randomised clinical trials that are used assessing drugs have also become a key piece for psychotherapy, although with a series of specific difficulties¹¹⁵:

1. ~ There are a series of common factors that are well-recognised in all psychological treatments, which are related to the therapeutic relationship (such as unconditional acceptance, the expectation of change, etc.) and are potentially important to the treatment outcome, as well as a series of active ingredients that are not easy to specify. Moreover, psychotherapies are constantly evolving, and new techniques are developed within the same approach or through approximations between different orientations. This can mean a source of innovation and flexibility in routine clinical practice in the interest of each patient, but in a clinical trial, treatment manuals that clarify exactly what has been done in the trial are essential.
2. ~ It is necessary to control the dependent variables of the therapist, especially their psychotherapeutic training and clinical experience, as well as their adherence to the treatment manual through independent evaluations.
3. ~ It is impossible to blind the treatment for whoever is administering it (psychotherapist), and it is relatively problematic to mask the active treatment condition for the independent external evaluator.
4. ~ The same psychological treatment can be provided in different formats (for example, individual or group), and the duration can also be variable, which should probably be evaluated separately, given that it could affect the outcome and efficiency.
5. ~ Several variables have been proposed for the patient (personality, biographical history, education level, etc.) or for the depressive disorder (type and severity, previous recurrence, etc.), which could have a greater likelihood of affecting the outcome of psychotherapy than the outcome of biological therapies.
6. ~ In recent years, clinical practice guidelines are consistent in recognising the efficacy of psychological therapies in the treatment of depression, especially those that have been specifically designed, such as cognitive behavioural therapy and interpersonal psychotherapy^{21, 29, 97}.

6.1. Cognitive Behavioural Therapy (CBT)

Cognitive therapy was originally developed by Beck and formalised at the end of the 70s in order to be applied to depression¹¹⁶ and it has become the most-frequently studied mode of psychological therapy in depression¹¹⁷. Intervention focuses on the modification of dysfunctional behaviours, distorted negative thoughts associated with specific situations and disadaptive attitudes related to depression. The therapist adopts an educational style and seeks the patient’s cooperation so that he can learn to recognise his patterns of negative thinking and re-evaluate this thinking. This approach requires that the patient practice his new skills between sessions through tasks at home and that he try new behaviours. It is commonly acknowledged that all cognitive therapy interventions include more or less behavioural techniques, which is where it gets the name of cognitive behavioural therapy.

Moreover, the CBT includes a range of different interventions that share the basic theoretical assumption that most human behaviour is learned. Thus, these interventions seek to implement certain skills in depressed persons, such as problem-solving therapy, assertive training or behavioural couples therapy. Behavioural activation is one of the ingredients of Beck’s cognitive therapy, which emphasises the relationship between activity and mood and which has been the object of renewed attention¹¹⁸. Nevertheless, these other interventions have been evaluated with less intensity than Beck’s cognitive therapy, and they are summarised briefly below.

CBT adopts a structured format, it is limited in time, and it is based on the cognitive-behavioural model of affective disorders. It has been evaluated in a diversity of contexts (primary care, specialised care, hospitalised patients), compared mainly with pharmacological treatment and with different patient samples¹¹⁹. The most frequent duration varies between 15-20 sessions of 50 minutes each and at a weekly frequency, although there are also studies that adopt briefer formats for less serious cases (from 6 to 8 sessions), and it is assumed that the duration of therapy can be extended in the event of greater severity or associated comorbidity.

NICE²¹ synthesises the available evidence based on 30 trials referring to patients whose depression varies from moderate to very severe, with the following conclusions:

1. ~ CBT is as effective as antidepressant drugs in reducing the symptoms at the end of treatment. While the effect is maintained for one year after ending CBT, the same thing does not always occur with antidepressants. This therapy seems to be better tolerated than antidepressants, especially in patients with severe or very severe depression, and its benefits are maintained to a greater extent than antidepressant medication.
2. ~ Adding cognitive behavioural therapy to antidepressant medication is more effective than treatment with antidepressants alone, especially in those patients with severe symptoms. In general, there is no indication that adding antidepressants to cognitive behavioural therapy is useful, although effects on specific symptoms, such as sleep, have not been explored. There is insufficient information to assess the effects of combined treatment on relapse rates.
3. ~ There is evidence coming from an wide study on chronic depression¹²⁰, that a combination of CBT and antidepressant medication is more beneficial in terms of response and remission than either of those treatments separately.
4. ~ In patients with residual symptoms who take antidepressants, adding CBT seems to be useful, given that it reduces the relapse rates during follow-up, although this advantage is not always evident at the end of treatment.
5. ~ With respect to the mode of providing treatment, there are data that suggest that group cognitive behavioural therapy is more effective than other group therapies, but there are few studies that compare the group mode and the individual mode of cognitive behavioural therapy to each other. A group mode of cognitive behavioural therapy based on mindfulness seems to be effective at maintaining the response in people who have recovered from depression, especially in those who have had more than two previous episodes¹²¹.

An important study subsequently published endorses CBT as a therapeutic strategy in resistant depression. In the STAR*D¹²² project, after an unsatisfactory response to citalopram, patients who accepted being randomly assigned to an alternative drug strategy or to CBT generally had a comparable outcome. Adding another drug produced a faster outcome than adding CBT to citalopram, although the switch to CBT was better tolerated than the switch to another antidepressant. The degree of acceptance of CBT (less than a third of patients) has been considered relatively low, although there are aspects of the experimental design that could have skewed the choice, as the authors themselves acknowledge, in addition to the fact that the cost of the therapy had to be paid for in the event that it was not covered by the patients’ health insurance, which would not occur in the case of pharmacological options.

Regarding recurrent depression, it has been seen that increasing the duration of CBT can reduce relapse and recurrence, especially in high-risk patients¹²³. This protector effect seems to be robust, regardless of whether or not CBT is provided alone or in combination with medication or if the combination is applied simultaneously or sequentially. Furthermore, another clinical trial reports a reduction of the relapse/recurrence rate from 72% to 46% (in a period of 2 years) in patients with multiple previous episodes by adding 16 hours of CBT to the usual treatment after remission¹²⁴.

In samples of patients with greater severity and comorbidity (more than 80% of the cases showed an additional diagnosis on Axis I or II of the DSM, 30% with melancholic depression), a controlled clinical trial with placebo showed that CBT reaches efficacy that is comparable to pharmacological treatment (which included dispensing suitably increased doses or augmentation, if necessary), although this degree of effectiveness seems to be linked to a high level of experience of the cognitive behavioural therapist¹¹⁹.

This recommendation of CBT as first-line treatment in severe depression expands prior recommendations referring to mild and moderate depression¹²⁵. It has been included in the decision algorithm of the clinical practice guidelines of Australia and New Zealand⁹² although they suggest drug therapy in advance, but not in the ICSI⁹¹ or NICE²¹ which preferably recommend combined therapy in depression that is initially shown to be severe.