This is the first study to prospectively assess the long-term effects of ayahuasca in participants grieving the death of a loved one. In this observational study, the primary result showed that the severity of grief significantly decreased after drinking ayahuasca, being maintained over 1 year of follow-up. The subgroup analysis performed in a representative subsample allowed us to infer that the significant differences in outcomes are due to the effect of their stay at the center and not the passing of time. The large effect size obtained at all point assessments on the primary scale clearly exceeds the mean effect size post-intervention (Hedges’s g = 0.41) and at follow-up (g = 0.45) obtained in the last meta-analysis focused on the effectiveness of current treatments developed to treat complicated grief and prolonged grief disorder (PGD) (Johannsen et al. 2019). Studies that have obtained higher effects sizes have applied specific protocols of intervention, which involved conducting from 12 to 25 psychotherapeutic sessions during periods of 3 to 9 months, or even longer (Bryant et al., 2014; Rosner et al. 2014; Boelen et al. 2007). We dutifully note that the Temple of the Way of Light is not conducting a specific program to treat grief symptoms, and that the maximum duration of the retreat was 1 month, during which ayahuasca was taken a maximum of 9 times.

It is well known that grief symptoms usually present a high rate of comorbidity with mood and anxiety disorders, influencing the quality of life of bereaved adults (Boelen and Prigerson 2007). In our sample, almost all the scales in SA-45 were above the score of 60 at baseline (percentile of 84) (Davison et al. 1997) suggesting likely problems with somatization, depression, obsessive-compulsiveness, anxiety, interpersonal sensitivity, phobic anxiety, and psychoticism. With the exception of phobic anxiety, all of these symptoms decreased significantly at all time points over follow-up to a level that is considered within the normal range in a non-patient normative sample (Davison et al. 1997). These results are in line with previous studies that found lower levels of psychopathology after drinking ayahuasca in first-time ayahuasca ceremony participants (Barbosa et al. 2009), as well as in long-term users as compared with control groups (Bouso et al. 2012; Dos Santos et al. 2016). Given that phobic anxiety is expressed through symptoms of uneasiness when in open spaces and crowds, the fact that we did not find significant differences in this scale after the retreat, but found differences throughout later follow-up assessments (T2, T3, and T4), could indicate a need for a period of time to adapt to the city environment after being in the jungle. However, phobic anxiety and psychoticism are on the threshold of being considered problematic areas, according to the follow-up assessments. As far as we know, this is the first article to look deeply into these psychopathological symptoms among a sample of grievers, so we cannot compare many of these subscales with other studies. Only one previous study has assessed a few of these psychopathological symptoms after participants received prolonged grief-specific cognitive-behavioral therapy (Rosner et al. 2014), an integrative cognitive-behavioral approach that includes structured exposures and cognitive restructuring. In this study, there were no differences in terms of phobic anxiety, somatization, and anxiety when participants were compared with a waiting list group.

The significant long-term improvements on all the scales of quality of life in our sample offer a broader picture of the impact of ayahuasca on the evolution of grief. Previous studies have found similar results in first-time ayahuasca ceremony participants (Barbosa et al. 2009), as well as improvement in psychosocial well-being in regular participants (Bouso et al. 2012; Dos Santos et al. 2016). However, last meta-analysis on psychological interventions for grief did not find statistically significant effects on health-related quality of life (Johannsen et al. 2019).

Although subjective responses about the persistent harms and benefits of ayahuasca experiences at the retreat support the overall outcomes, we must bear in mind that a large proportion of the participants expected that ayahuasca ceremonies would improve their grief symptoms, which could promote suggestibility and enhance positive outcomes. The fact that most of them affirmed that ayahuasca had a positive effect in terms of mental health and spirituality led us to ponder the role that spiritual coping played in the adjustment of bereavement in our sample. Previous studies have shown that spirituality can play a centrally important role in helping individuals adapt to the loss of a loved one (Wortmann and Park 2008; Hays and Hendrix 2008).

In our sample, 78.4% of participants described an ayahuasca experience that directly affected their grief process. Although the content analysis of participants’ ayahuasca experiences is pending publication, a previous study described emotional confrontations with the reality of the death, the reviewing of biographical memories, and a reencounter with the deceased, as common ayahuasca experiences among grievers (González et al. 2017). The pharmacological effects of ayahuasca have been described extensively (Dos Santos et al. 2012, 2013; Riba et al. 2001, 2011), including a dose-dependent curve with a slow onset during the first 45 min, a peak in the experience at around 90 min, and slow decrease in the effects from 150 to 250 min. Under the effects of a medium or high dose, the peak of the experience could be similar to the immersion used in virtual reality exposure therapy (VRET), which allows individuals to bypass symptoms of experiential avoidance (Gonçalves et al. 2012). Other studies have compared acute ayahuasca effects with a controlled exposure to autobiographical material (Domínguez-Clavé et al. 2016). This acute confrontation with difficult memories, thoughts, and feelings could promote increased acceptance, leading to psychological modifications that are observable beyond the time frame of the acute inebriation (Soler et al. 2018). Although this is the only study that has assessed the modulation of acceptance (or experiential avoidance) after an ayahuasca experience over the course of 1 year, our results are in line with other studies that have found an increase in acceptance to result from an ayahuasca experience (Domínguez-Clavé et al. 2016; Soler et al. 2018). Nevertheless, around 150 min after ayahuasca intake, the intensity of the effects begins to decrease, allowing a healthy distance from the psychological material and leading to a reflective phase (Kjellgren et al. 2009). During this phase, decentering could improve (Fresco et al. 2007). Gains in the ability to decenter are in line with other studies that have found an increase in decentering abilities following ayahuasca intake (Domínguez-Clavé et al. 2016; Soler et al. 2016; Sampedro et al. 2017), and in those with more than 15 ayahuasca experiences compared with those with non or few experiences (Franquesa et al. 2018). However, contrary to a previous study (Sampedro et al. 2017), the gains in decentering in our sample persisted over a year of follow-up.

These results are also in line with previous studies that show a positive relationship between high levels of acceptance and high levels of decentering (Hayes et al. 2017). In our sample, we observe that increased acceptance and decentering following the retreat have a significant correlation with improvements in the severity of grief. Acceptance is considered a key mechanism of change in complicated grief treatment (Glickman et al. 2017), a form of cognitive-behavior therapy with elements of interpersonal psychotherapy and motivational interviewing (Shear et al. 2016). On the other hand, decentering is a mediator in the metacognitive approach to treating grief (Wenn et al. 2015), a form of psychotherapy that uses detached mindfulness to alter dysfunctional thinking styles by helping individuals to understand their “thoughts about thoughts.” Accommodation (new learning) is a psychological process that has been associated with an ability to engage with traumatic experiences when it is combined with more acceptance and decentering (Hayes et al. 2017). Accommodation provides an opportunity for constructive processing and meaning making regarding the experience (Neimeyer 2019). Moreover, other kinds of experiences, such as having a reencounter with the deceased (González et al. 2017), could modulate the internal working model of the deceased and the attachment style maintained with him or her after death. Future studies should explore more potential mechanisms of change in order to reach a better understanding of the therapeutic potential of ayahuasca among the bereaved.

The study has some limitations. First, the naturalistic design of the study does not allow us to isolate the effects of ayahuasca from the other diverse variables in the setting, which include the healing work of the Shipibo shamans during the ayahuasca ceremony. In fact, the separation of the effects of ayahuasca from the healer’s work is not conceived of within the Shipibo traditional medicinal framework, in which participants drank the ayahuasca. Moreover, without a placebo group it is not possible to know if ayahuasca was truly responsible for the improvement in grief symptoms, as it is not possible to draw conclusions about causality. Second, the lack of scientific evidence on the stability of ayahuasca alkaloids over time does not guarantee similar concentrations when participants took ayahuasca in the center and at the time, the samples were analyzed. Moreover, the lack of knowledge about the doses used throughout the ceremonies by each participant prevents us from assessing to relate the improvement in the severity of grief in relation with the doses of DMT or betacarbolinnes ingested. Third, the sample size is relatively small and there were a large number of participants that did not complete all the follow-up assessments. This limitation can be extended to the subgroup analysis, which was conducted to control for the effect of time passing. Fourth, nine participants attended psychotherapy over the year follow-up, four took ayahuasca again to treat grief symptoms, and one participant took medication, so we cannot attribute the maintenance of the improvement in grief symptoms exclusively to the persistent benefits of the ayahuasca experiences at the retreat. Fifth, we did not assess Prolonged Grief Disorder or Persistent Complex Bereavement Disorder, having used self-report measures rather than clinical interviews. This, along with the fact that the sample was heterogeneous, in terms of the causes of loss and the amount of time that had passed since the loss, makes the extent to which subgroups profited differently from ayahuasca uncertain. Lastly, we used a direct method to recruit participants who were willing to participate in ayahuasca sessions, which may limit the generalizability of our results to treatment-seeking bereaved people in general or whose able to respond an online survey. To reach stronger conclusions, large controlled cohort studies or randomized, placebo-controlled trials conducted with adequate samples of participants will provide evidence of causality.