A needs assessment was performed between September 2, 2019, and March 31, 2020. First, existing literature was consulted to analyze the health problem of participating in chemsex to get a clear picture of the associated health risks and risk behaviors. Second, the possible determinants of each of the risk behaviors were identified through in-depth interviews and behavior change theories (BCTs). Our intervention did not aim to include determinants on the environmental level (eg, health care providers and peers), which is why only determinants on the individual level were identified.

The literature search involved articles retrievable via the PubMed, ScienceDirect, and Google Scholar databases. The following search terms were used: “chemsex,” “drugs,” “risks,” “health,” “behaviour,” “motivation,” “MSM,” “sexualized drug use,” and “men who have sex with men,” as well as combinations of these terms. Relevant papers were selected and reviewed. In addition, the reference lists of the selected articles were reviewed manually to identify other relevant articles.

Intervention mapping uses a multitheory approach because different theories may focus on specific aspects of behavior or behavior change [43,44]. The BCTs we selected to guide the selection of determinants for our intervention were the social cognitive theory (SCT) [45,46], the theory of planned behavior (TPB) [47,48], and the precaution adoption process model [49]. The rationale for choosing these BCTs is based on existing literature and described in the Results section.

After analysis of the problem, data were collected through 20 semistructured in-depth interviews with Flemish chemsex participants [6]. The aim of these interviews was to gain more insight into the specific Flemish chemsex context and also complement the results of the analysis of the problem by exploring the determinants of chemsex behavior and the needs of the key population. The interview guide explored current risk reduction practices (before, during, and after a chemsex session) and the needs of chemsex participants. Specific methods, participant characteristics, and detailed results have been published in a dedicated article [6]. The results of the interviews enabled us to assign relevance to the health risks of MSM engaging in chemsex and the underlying determinants. The results also helped us to formulate achievable performance objectives (step 2 of the IMP) based on the preventive measures MSM take when participating in chemsex.

To evaluate the first version of the Budd app, we performed a mixed methods usability evaluation. Think-aloud testing was followed by administering a usability questionnaire and completed by conducting semistructured interviews after an app testing period of 2 weeks. Our objectives were to (1) identify design, usability, and functionality issues; (2) assess acceptability; and (3) assess satisfaction with the intervention components.

We collected feedback from 8 chemsex participants, of whom 4 (50%) were recruited from the respondents who participated in the in-depth interviews (step 1) and ticked the box on the informed consent form indicating that, at a later stage, they would like to provide feedback on the developed intervention. The remaining (4/8, 50%) respondents were recruited through pre-exposure prophylaxis (PrEP) or HIV and STI consultation at ITM.

The eligibility criteria included being aged at least 18 years; self-identifying as a male member of the lesbian, gay, bisexual, transgender, queer, and similar minority community; being able to understand, and express oneself in, Dutch or English; having intentionally used drugs to have sex within the past 12 months; and owning a smartphone.

The study was conducted at ITM by CH between February 1, 2021, and April 30, 2021. At the start of the study, the procedure was explained, and the participants signed the informed consent form. The researcher helped each participant install the app on their smartphone. First, participants completed a preset list of different tasks of varying levels of complexity that cover the full range of functionalities of the app (Multimedia Appendix 2). The researcher was not allowed to help participants complete the tasks. To evaluate usability, the think-aloud method was applied to understand respondents’ thoughts as they occurred and attempted to cope with the issues they encountered [51]. After performing all of the tasks, the study participants filled out the 10-item system usability scale (SUS) questionnaire [52]. The SUS is a widely used validated method for assessing the usability of a system [53]. Next, the study participants tested the Budd app in their own environment for 2 weeks. After this test period, a follow-up interview took place at ITM to assess the (1) design, (2) usability, (3) satisfaction with the included intervention components, and (4) acceptability. The interview guide can be found in Multimedia Appendix 3.

A pragmatic analysis approach was used to analyze the usability testing and follow-up interviews [54]. In both exercises, the researcher took extensive notes [55,56]. This method was chosen to save time and resources in data processing and subsequent analysis [56]. During the usability test, the researcher collected data by observing the study participants carrying out the tasks while also writing down the thought processes and feedback they shared. During the usability test as well as the in-depth interview 2 weeks later, notes were categorized in a template with subject headings created beforehand [57]. For the usability test, the notes were structured into the categories usability issues and ideas for improvement. The usability issues were further specified for each intervention component. For the follow-up interview, the researcher registered the main outcomes for each of the 4 topics by writing a summary for each topic discussed. Notes from both studies were analyzed by conducting a deductive thematic analysis [58].

After completing the tasks, all participants completed the SUS questionnaire. The scale consists of 10 questions measured using a 5-point Likert scale (ranging from strongly disagree to strongly agree) and is based on 3 usability criteria: effectiveness, efficiency, and user satisfaction [59]. The final score from the SUS can range from 0 to 100 points. A score of >68 points is above average and indicates adequate usability. In this study, the individual SUS scores of the participants were summed and then divided by the number of participants to obtain a mean usability SUS score.

We found that participating in chemsex is associated with a wide range of health risks and related risk behaviors. Although the majority of chemsex participants do not identify with problematic use and experience fewer minor consequences [10,11], there is a group of chemsex participants who do experience harms resulting from their participation. The health problems associated with chemsex can largely be divided into 4 categories. Each category is explained in more detail in the following sections.

The first cluster of chemsex health problems cited in the literature involves drug-related physical harms. An acute risk, frequently reported, is drug overdose [8,60]. The degree of overdose and its effects can range from aggression, heart palpitations, breathing difficulties, panic attacks, delusions, hallucinations, dehydration, overheating, memory loss, and unintentional injury to loss of consciousness and even death [8,60]. Depending on the type of drug, there are also many potential long-term risks, including substance dependency, alterations of cognitive functions, heart disease, psychosis, high blood pressure, movement disorders, and weight loss [2,61].

The risk behaviors contributing to these health problems are a variety of high-risk drug use practices. Polydrug use is the norm for many MSM engaging in chemsex, although percentages vary between 7% and 78.3%, depending on the study locations and definition used (starting from 2 or 3 substances) [2,13,62-64]. Other important risk behaviors include the use of crystal methamphetamine [29,65] and new psychoactive substances [66], injecting drug use during sex (known as “slamming”) [18,29,67,68], sharing user equipment (snorting devices and injecting equipment) [18], using high doses [2], extensively redosing [2,69], and going multiple days without sleep [67].

The second cluster of reported chemsex health problems is related to sexual health. Taking drugs before or during sex helps MSM to experience sex without inhibitions [70,71]. This ensures that people are prepared to participate in a wide range of sexual acts, which sometimes go beyond their limits [2]. Chemsex has been associated with high-risk sexual behaviors, including group sex; condomless anal intercourse; fisting; bondage, discipline, sadism, and masochism; having a high number of sexual partners; long sexual sessions that can lead to rectal and penile trauma; and transactional sex [2,7,18,26,63,66,72,73].

MSM who engage in chemsex are more likely to contract HIV, hepatitis C, and other STIs than those who do not engage in chemsex [18,62,74]. Taking drugs can also lead to sleep problems and memory impairment, which, when combined with prolonged sex sessions, can cause people to forget about taking antiretrovirals for HIV management or PrEP [75]. Certain drugs have also been shown to interact with antiretroviral drugs, which is reflected in a negative impact on clinical HIV outcomes [75].

Chemsex participants may also experience difficulty in having sex without drugs because the drugs used hugely increase sexual drive and desire [2]. In this context, being unsatisfied with one’s sex life is associated with participating in chemsex [60]. The strong feelings of intimacy and connection experienced during a chemsex session disappear after the drugs wear off, contributing to the level of dependency in which a large proportion of chemsex participants are in search for connection [76]. Some chemsex participants also report being the victim of nonconsensual sex while under the influence of drugs [2,60]. A recent study among MSM in Amsterdam showed that 41.4% of the participants who engage in chemsex reported passing out and not remembering what happened while under the influence of drugs [77].

The third cluster of chemsex health problems being increasingly reported in the literature involves the adverse impact of chemsex on mental health [17,78,79], although research on the topic remains limited. A recent systematic review found a positive association between chemsex and depressive symptoms, anxiety, and dependence on drugs or other substances [80]. The practice of slamming, especially, is associated with a greater number of mental health symptoms [29,66].

The acute mental health risks of participating in a chemsex party are caused by the substances used. Depending on which drugs are used, risks can include agitation, confusion, feelings of anxiety, aggression, restlessness, and irritability [2]. With crystal methamphetamine specifically, there is a risk of extreme paranoia and panic attacks [29,70]. People often experience poor concentration, difficulty with information processing, and feelings of anxiety and irritation in the days after a chemsex party (comedown) [2,6]. These feelings are a result of using stimulant drugs (crystal methamphetamine, ecstasy, and mephedrone) [81]. In the long term, drug use can also lead to anxiety disorders, depression, psychosis, memory problems, and personality changes [2,81].

Anecdotal evidence at our clinic at ITM further indicates an impact on other areas of chemsex participants’ lives. Some state that they struggle with relational, financial, and work-related factors as a result of their participation in chemsex. There is limited evidence fully identifying the social impact that chemsex behaviors have on chemsex participants’ lives. In a study among chemsex participants in Dublin, 25% of the participants reported that chemsex had a negative impact on their lives [15]. The impact on the work situation seems to be most pronounced during the comedown period because people experience difficulties in working efficiently, concentrating, and feeling motivated. As a result, chemsex participants are more likely to perform poorly at work or report sick on the days after a chemsex party [2,18]. Chemsex participants who use methamphetamine report that the use of the drug reduces their ability to fulfill daily tasks [82].

The determinants of chemsex participation were identified and categorized on the basis of a literature review guided by the selected BCTs and in-depth interviews.

Awareness is defined as the degree of knowledge and understanding of one’s own unhealthy behavior [49]. The findings from the interviews indicate that the effects of drugs can greatly reduce the awareness of one’s own risk behavior during a chemsex session [6]. Respondents report losing themselves completely in the intense feelings of sexual arousal and disinhibition [6]. Only when a person is also aware that they are engaging in risk behavior (eg, know how much time they leave between 2 dosages and know their own [sexual] boundaries) can they consider changing this behavior. Awareness of one’s own risk behavior is seen as an essential first step in the process of behavior change, according to the precaution adoption process model [49]. In an attempt to increase awareness, we included this essential determinant in the development of our intervention.

The transfer of knowledge is often a key element in traditional health education [50]. Information about health behavior and the health risks of certain behaviors is necessary to enable behavior change. The in-depth interviews strongly emphasized this need for reliable information [6]. Particular attention needs to be paid to information about chemsex drugs, the effect of combined chemsex drugs, how best to act in the event of an emergency, and an overview of existing (drug and sexual) health care services. Respondents indicated that a lack of information often hindered them in adopting risk reduction practices [6]. This need is also reflected in studies conducted in other countries, such as a UK study that showed a need for reliable and nonjudgmental information about safe drug practices [8] and the European Men-Who-Have-Sex-With-Men Internet Survey 2017, which found that respondents scored low on HIV and STI transmission knowledge, postexposure prophylaxis knowledge, PrEP knowledge, HIV test and treatment knowledge, and hepatitis A and B test knowledge [83]. Knowledge is thus a presumably important determinant in practicing safer chemsex.

According to the SCT, human behavior is, to a large extent, determined by the expectations one has of a behavior [46]. One of these expectations is the person’s belief about their ability to successfully influence their environment [46]. This concept of self-efficacy is a central concept in the SCT, the importance of which is widely recognized. In the context of our intervention, the SCT is cited in several studies as a suitable model for behavior change among individuals who take drugs. Numerous studies have shown that self-efficacy plays an important role in abstinence, substance relapse, and adopting harm reduction measures [84,85]. More specifically, a UK study shows that engaging in chemsex is associated with lower sexual self-efficacy [86]. A study among MSM who had tested positive for HIV and who were using methamphetamine shows that being convinced to be able to say no to drugs (drug assertiveness skills) was associated with reduced frequency and amount of drug use as well as less sexual sensation seeking and unprotected sex [87]. Higher self-efficacy also has a direct positive effect on HIV treatment adherence [88,89] and PrEP adherence among MSM in the United States [90].

The TPB states that the best way to predict behavior is to ask people whether they intend to exhibit that behavior: the behavioral intention [47,48]. According to Fishbein and Ajzen [47], this behavioral intention is determined by 3 factors: the person’s own views (attitude), the views of others (subjective norm), and the estimation of the person’s own possibilities of carrying out the behavior (perceived behavioral control, based on the concept of self-efficacy mentioned previously).

The TPB is one of the most widely applied behavior change models and has been shown to be effective in explaining and predicting a wide range of health behaviors [91]. Several studies report that the TPB is an appropriate model for explaining, understanding, and predicting a variety of chemsex-related health behaviors, some of which we identified in step 1 of the IMP. These include safer sex behaviors [92], condom use [93,94], therapy adherence [95], intention to enter drug and alcohol treatment [96], and the use of safe injecting procedures [97]. Furthermore, a meta-analysis supported the use of the TPB as a valuable framework for designing interventions to reduce heterosexual risk behavior [98]. Therefore, we considered the TPB suitable to guide us in the development of our intervention.

The participants performed various tasks within the Budd app, providing us with a rich data set containing their feedback. The usability issues and ideas for improvement mentioned by the respondents were classified according to their intervention components. All results can be found in Multimedia Appendix 6.

The average usability score was 76.88, which indicated the need to make only minor usability improvements to the first version.

Respondents used the app for different purposes; for example, some (4/8, 50%) of them found the informative articles especially useful because they contained practical, to-the-point, and clearly defined information. Of the 8 respondents, 2 (25%) found the process of planning chemsex sessions helpful because it increased awareness of participation before attending a session, whereas 2 (25%) found this component to be less meaningful because they usually do not know in advance when they will participate in a chemsex session as these sessions happen unexpectedly more often than not or at the last minute.

Respondents also provided feedback on potential improvement of the app components. Some (5/8, 63%) of them mentioned that basic information on different substances was lacking, whereas others (2/8, 25%) stated that information on specific new psychoactive substances and the risks of slamming was missing. Respondents also stated that the section on drug combinations could be worded more in layman’s terms. When checked in on the app at a chemsex session, respondents found it inconvenient that they could not easily return to the rest of the app because they had to check out first.

Finally, respondents also shared ideas for adding intervention components. These focused mainly on expanding the substance-related–information section, giving a voice to other chemsex participants, adding ways to keep track of thoughts, and increasing awareness of time during a chemsex session.

The main usability issues had already been addressed in the usability test (“Performing tasks”). The answers to the questions about usability were therefore rather limited. The 2-week test period did lead to some suggestions to improve usability that were not noticed during the first test. These included the following: it is inconvenient that there is no chemsex session overview, public holidays are not visually indicated in the calendar view, have the weeks start on Monday when “adding a new event,” and the red bar bearing the legend “Call emergency service” overlaps with text at the bottom of the screen.

Three factors were surveyed to assess acceptability: frequency of use, whether people expect to continue using the app (intention to use), and whether they would recommend the app to others. The frequency of use reflects the same finding as the preference for certain intervention components. Some (2/8, 25%) of the respondents only used the app just before and during a chemsex session, whereas 13% (1/8) used it primarily to read all the information, and 38% (3/8) used the app very regularly for the information it provides and to support their participation in chemsex. All respondents stated that they intend to continue to use the app when it is generally available. The reasons for this were varied and included the following: will use it (1) depending on frequency of participation, (2) because it has a scientific basis, (3) to monitor substance use, (4) to monitor participation in chemsex, (5) as a personal safety tool, and (6) for the educational aspect. All respondents were also willing to recommend the app to others but again for different reasons: to (1) prepare people just starting to participate in chemsex, (2) make people more conscious about their participation in chemsex, and (3) inform people about assessing the interaction of drugs using the drug combination tool. Of the 8 participants, 3 (38%) stated that they hoped that it would become a common tool in the community so that the use of the app during parties would become easier.

In general, the respondents were very positive about the design of the app. This may also be partly due to the fact that we had already carried out a small-scale design review with 5 respondents at the start of the development process. The respondents found the layout and structure to be clear. However, the respondents felt that some parts of the interface could be more consistent; for example, the placement of certain buttons and whether pop-ups should be used. The font, according to the respondents, is easy to read. The use of color was also well received, with respondents describing the colors used as neutral, calm, and generating the feeling that “we take care of you.”