The hypothesis that cHAT would improve treatment retention and completion was based on recently published results from a study by our group on a sample of 108 patients, showing that the subjects in the cHAT programme (n = 65) were more likely to complete their treatment than those attending TAU-only (adj. OR 8.4, 95% CI 2.7–26.4; P < 0.001) [19]. The scope of the RCT design was to compensate for the lack of randomization in the previous study, which limited the generalizability of the results to a population potentially more motivated to participate in the cHAT programme. In the current study, we found no association between cHAT and treatment completion and thus were unable to confirm the previously shown statistically significant association of cHAT with treatment completion. We suspect that the reduction in sample size from n = 50 to n = 37 subjects, and the fact that only 2 patients (11%) completed the 12 cHAT sessions, were the main cause of the lack of statistical significance in the results. In our clinical routine, patients can request to participate in the cHAT programme through their therapists or can be referred by them; hence, the findings from this study remain relevant for clinical application.

Using dropout as a standard measurement and comparing it between studies is complicated by the fact that a unique and concordant definition of dropout is lacking [13]. Nevertheless, patient failure to complete therapy, usually defined as dropout, is a commonly used evaluation measure of the SUD treatment process. In our study, 24% of patients dropped out of treatment, which is below the level that we observed in our previous investigation (60%) and the rate often reported in studies of addiction (more than 50%) [4, 8, 13, 14].

In the TAU-only programme 42% of subjects were relocated to another residential treatment, compared to 28% in the cHAT programme, but this was not statistically significant. A previous study found that cHAT is a good strategy for retention in treatment [19], but this was not confirmed in this underpowered RCT study. Neither did we find the cHAT programme to be associated with longer times in treatment, as previously reported [19]. The optimal duration of treatment is debatable, and it depends on both the treatment method and the problems of the individual patient [1], but 90 days is often identified as the minimum period for effective treatment [4, 7, 46]. In our previous investigation [19], we found that subjects participating in the cHAT programme remained in treatment for a significantly longer period than those in the TAU-only group (mean 141 days vs. 70 days) and that they were more likely to remain in treatment for 90 days or more. No association between participation in the cHAT programme and longer time in treatment was observed in the current study, although in both treatment groups the average time in treatment was more than 90 days.

To our knowledge, there is no empirical evidence on the minimum number of cHAT sessions required to achieve a treatment benefit. In our protocol, the first four sessions of HAT were introductory, while the following eight were targeted at the therapeutic goal. In the study, most of the subjects attended fewer than eight sessions, indicating an overall low commitment to the cHAT treatment. We decided to analyse whether there was a specific number of cHAT sessions that would give a better outcome (lower dropout rate) and observed that 75% of subjects who had attended fewer than eight cHAT sessions dropped out of treatment, whereas only 25% of subjects who had attended more than eight sessions did so. Despite the cohort being too small to run a statistical analysis, this observation might indicate a need to provide a minimum of eight cHAT sessions in cHAT programmes for the treatment of patients with SUD. The relevance of the number of cHAT sessions for a certain outcome should be assessed individually (e.g. retention in treatment, treatment completion, symptoms reduction) on a larger study sample, and the programme designed accordingly.

Females in treatment for addiction constitute a marginalized group in SUD treatment, where the gender distribution is normally two-thirds male and one-third female [47, 48]. The finding that in the cHAT group females attended a higher number of cHAT sessions than males suggests further investigation of the hypothesis that the HAT programme is a promising treatment option for SUD females.

Strengths and limitations

The strength of our study is the use of a randomized parallel-groups controlled design. Unfortunately, the sample size was drastically reduced during enrolment, study and final analysis, compromising the statistical analysis and increasing the risk of a type II error. The therapeutic efficacy of the cHAT treatment reported here may be an underestimation. This seems probable considering that our data, at least in relation to treatment completion, showed the same trend as that observed in our previous study where patients in cHAT were more likely to complete their treatment than subjects in the TAU-only group [19].

The RCT design in a study assessing an intervention such as cHAT, where a placebo design for the control group is not feasible, and with a clinical population characterized by high rates of dropout and non-compliance with treatment [49], was more difficult to apply than with the non-random allocation of our previous study. Indeed, several patients withdrew their consent because they were not assigned to the treatment programme in which they wished to participate. Moreover, a high number of subjects (67%) in cHAT may have attended an insufficient number of cHAT sessions (cHAT < 8). Notably, the majority of subjects had a diagnosis of multiple drugs and psychoactive substance use, and at least one of concurrent psychopathology, constituting a clinical population with severe SUDs, and such people have notoriously high rates of dropout and low treatment compliance [49].

Both TAU and cHAT are person-centred treatment programmes tailored to individuals’ specific problems and treatment goals. This implies that the therapy varies between patients in terms of goals and duration of treatment. We did not control for the number of treatment sessions (either for TAU or for cHAT), which is a limitation. Another limitation is the lack of a control group for the complementary activity, for example, dog-assisted therapy or gardening, which would have specifically controlled for the horse contribution. Retrieval of information from the hospital clinical records can potentially be affected by subjective interpretation. This is a limitation in our study, but we controlled for the retrieval of the outcome variables by having this done by personnel blind to the experimental conditions and without any affiliation to the authors or study site.

Further research

In a recent review of animal-assisted therapy for SUDs, Klemetsen and Lindstrøm [50] have highlighted the lack of studies with a strong methodological design, advocating for the urgency of more randomized and controlled studies. Very few peer-reviewed studies investigate HAT for SUD treatment, and none, to our knowledge, assess retention in treatment and employ an RCT design [18,19,20,21].

This study did not find a statistical significant effect of cHAT on SUD treatment completion, but the possibility of a type II error cannot be ruled out. Future RCT designs on cHAT with treatment completion as outcome variable should include a bigger study population to increase the statistical power. This would most likely promote more reliable results and clear conclusions on the utility of cHAT on SUD treatment completion.

Conducting a study with an RCT design on HAT in a SUD population is difficult and requires substantial resources. Furthermore, it is reasonable to assume that cHAT is more effective for patients actively seeking this treatment than with those not wanting it.

In the future, we hope to see RCT studies on HAT with sufficient statistical power, using outcome variables with large effect sizes. RCT designs give good causal indications, provided they have sufficient time, funding and statistical power. Future studies should control for the number of cHAT sessions or establish a pre hoc minimum number of sessions. We used treatment completion as the outcome variable, but future research should include additional outcome variables specifically related to HAT. In a qualitative study, the participants reported that HAT facilitated positive attachment, reflective functioning, self-efficacy and emotional regulation [44]. All of these, or other outcome variables theoretically associated with HAT, should be investigated. The research literature on HAT would also benefit from studies using longitudinal prospective cohort designs. This observational design cannot establish causation, but it was easier to implement than RCT. A prospective ‘pre–post’ cohort design with repeated measures, for example, could give important indications of the psychometric properties that seem to change over time with HAT, and for whom.