The combined search strategies identified 958 eligible records that were screened for inclusion in the study. Of the 14 potentially eligible studies, 12 full papers and two abstracts were obtained. Of these, ten studies met the inclusion criteria (Fig. 1). The abstract Lim et al.  included in Free et al. (2013)  review was excluded when the full paper Lim et al. (2012) revealed the intervention group received email and SMS. Similarly, Jones et al.  was published in full as Jones et al. , with the latter used in this review. Of the ten included trials, there were three intervention categories: 1) promotion of uptake of sexual health services, including reminders to attend a clinic 2) reduction of risky sexual behaviours and 3) reduced recall bias in reporting sexual activity.
Participants & characteristics of studies
The 10 trials included 16773 participants. Samples ranged from 52 to 7606 participants. Seven trials used a 2-arm design, two a 3-arm and one a 5-arm trial. All trials sought to address STI related issues with two studies focusing on increasing the uptake of testing [22, 23]; two focused on clinic re-attendance [24, 25]; four focused on risk reduction through sexual behaviour change [26–28] one focused on knowledge acquisition and risk reduction through sexual behaviour change  and one focused on reducing the recall bias when reporting sexual activity . Trials were conducted in high and low-income countries with at-risk populations.
The interventions are described in Table 1–4. For the two studies focusing on increasing the uptake of STI testing one used informational and motivational SMS , while the other used a video on a mobile device versus the standard paper-based protocol . SMS reminders were used for the clinic re-attendance trials [24, 25]; one with and without financial incentives . Risk reduction through behaviour change was trialed using SMS , video versus SMS , informational SMS  and informational SMS with theory based feedback and goal setting . One trial focused on knowledge acquisition and risk reduction through behavior change used SMS designed for the target population . Finally, one data collection study compared SMS to paper-based and online collection of sexual health information . The maximum number of behavior change techniques employed in interventions was four, the median number of behavior change techniques employed was two. Three interventions reported being developed based on behavioral theory.
Heterogeneity in interventions and trial outcome assessment and reporting did not allow for meta-analysis.
The trials reported between one and five outcomes. For primary outcomes, two trials reported outcomes related to clinic attendance [24, 25]. One trial reported uptake of sexual health services . There was also a trial that reported timeliness, completeness and response rate for the use of SMS to collect sexual health information . In regards to secondary outcomes, one trial reported uptake of HIV counselling and testing . Condom use was a common outcome measured among three of the four risk reduction trials [21, 28, 29]. In addition, sexual health knowledge and recent STI testing were also measured . Furthermore, early resumption of sexual activity post circumcision was also reported in one risk reduction trial . Four studies reported measures of acceptability of their interventions [26, 21, 28, 30].
The assessment of study quality is reported in Table 5. No trial had a low risk of bias for all quality criteria.
We report the risk ratios for primary outcomes and secondary outcomes. See Tables 6 and 7.
Uptake of use of sexual health services including increasing testing and clinic re-attendance
Two trials showed statistically significant increases in clinic attendance in participants receiving clinic reminder SMS compared to controls [24, 25]. Odeny et al.  noted a significant decrease in patients that failed to return for a clinic visit (intervention group were more likely to return) after male adult circumcision, relative risk (RR) 0.86, 95 % confidence interval (CI) 0.74–1.00. Downing et al.  showed that SMS reminders quadrupled re-testing for Chlamydia compared to controls (RR 4.5, 95 % CI 1.05–19.22). SMS reminder plus incentives had a similar effect as SMS reminders alone. Shahkolahi  conducted a 2-arm trial to improve rapid HIV testing in a hospital Emergency Department using videos, a mobile application and paper-based intervention. The authors reported that there was a statistically significant increase in uptake of HIV testing among intervention participants exposed to the mobile application, however, a full paper was not available for this study and risk ratios could not be calculated.
One 5-arm trial compared the use of motivational or informational SMS to improve uptake of HIV counselling and testing . Intervention participants either received 3 or 10 motivational/informational SMS. Receipt of informational SMS was not associated with a statistically significant increase in uptake of HIV counseling (RR 0.94, 95 % CI 0.81–1.09 and RR 1.02, 95 % CI 0.89–1.17 for 3 and 10 SMS respectively). However, study participants who received either 3 or 10 motivational SMS were less likely to take up HIV counseling and testing (RR 0.86, 95 % CI 0.73–1.00 and RR 0.8, 95 % CI 0.69–0.93 for 3 and 10 SMS respectively).
Reduction of risky sexual behaviours including knowledge acquisition and behaviour change
There were no studies that reported primary outcomes in relation to reduction of risky sexual behaviours.
None of the four trials showed statistically significant changes in sexual health behaviours. Gold et al.  explored the use of SMS to increase sexual health knowledge and intervention participants scored significantly better in their sexual health knowledge test (RR 1.75, 95 % CI 1.11–2.77) compared to the control group. There were no statistically significant changes in ‘always using condoms in the past 6 months’, (RR 0.87, 95 % CI 0.62–1.24).
Jones et al.  compared the effectiveness of HIV prevention messages delivered to smartphones either as weekly messages or through a soap opera video format over a 12-week period. There were no reported statistically significant differences between the two approaches (p = 0.39), although reductions in self-reported risky sexual behaviour (p <0.001) were reported in each arm compared to baseline at 3 and 6 months’ post intervention. Participants in the trial wanted to continue to receive the videos and reported they could relate to the characters.
Odeny et al.  assessed the impact of an SMS intervention to deter early resumption of sexual activity among men who had recently been circumcised. The authors did not find a statistically significant association between receipt of SMS and early resumption of sexual activity (RR 1.13, 95 % CI 0.91–1.38).
Suffoletto et al.  investigated the effect of an SMS intervention program to reduce risky sexual behaviour among young women attending an emergency department. No statistically significant differences between intervention and control arms were found for condom use with last vaginal sex (RR 1.4, 95 % CI 0.68–2.88) or for condom use with vaginal sex in the past 28 days (RR 1.4, 95 % 0.49–4.00). In terms of acceptability of the intervention, of the participants who completed the 3-month follow up, all stated that they found the SMS “very informative and very useful.”
Delamere et al.  assessed the effect of a 3-month SMS intervention to improve condom usage among young people attending a young person’s clinic. Participants in the intervention group were reported to be almost four times as likely as controls to have changed sexual partner during the study period (RR 3.65, 95 % CI 0.95–14.05), and twice as likely to have unprotected sex, (RR 2.03, 95 % CI 0.47–8.81), but neither result was statistically significant. In terms of acceptability, among intervention participants who were interviewed, 87.5 % reported the text messages useful in their decision making to use condoms, with 19 % of the cohort forwarding SMS to friends. All messages were rated as good, very good or excellent.
Sexual health data collection to reduce the recall bias when reporting sexual activity
Lim et al.  assessed three methods of sexual health data collection: paper, SMS and online diaries. They found that of the diaries submitted, 80 % of SMS diaries were submitted on the correct day in comparison to 63 % of online diaries.
Lim et al.  reported 14 measures of acceptability comparing SMS, online and paper diary collection, of which 13 were not statistically significant. The sole statistically significant measure demonstrated that participants were more likely to be uncertain about completing SMS diaries compared to online diaries (p = 0.047).
Finally, no subgroup analyses were conducted due to the low number of included studies in this review.