Why SMS still belongs in digital trials

When building a digital trial, SMS is easy to overlook. Compared to mobile apps and web platforms, it can feel like an afterthought. There is no rich interface, no video support, no interactive forms. But SMS still earns its place, especially in studies where not everyone is comfortable with apps or has consistent access to Wi-Fi, and there is real trial evidence behind that, not just intuition.

What a randomised trial actually found

A randomised controlled trial of an SMS support programme for young adults with type 2 diabetes, TEXT2U, is a useful reality check on both what text messaging can and can't do.

A higher proportion of the intervention group achieved 100% attendance (12/21, 57%, vs 5/19, 26%, for the control group). Middleton et al., TEXT2U randomized controlled trial

That's more than double the attendance success rate from a channel with no app, no login, and no interface to speak of. The same trial found no meaningful difference in blood sugar control, weight, or cholesterol between groups, and early gains in patient empowerment scores faded by the end of the study. Over 90% of participants said they'd recommend the programme to others. Read together, the honest conclusion is that SMS is genuinely effective at keeping people showing up, but it isn't a substitute for the clinical intervention itself. It's an engagement layer, not a treatment.

Why SMS holds up

Reach. Almost every mobile phone can receive a text message regardless of age or operating system. For participants who don't want to download another app, who have forgotten their login credentials, or whose device is too old to run the latest version, a text message is a reliable fallback.

Open rates. Unlike emails, which disappear into crowded inboxes, text messages are usually seen within minutes. That makes them well-suited to reminders, time-sensitive nudges, and quick check-ins.

Inclusivity. Older adults, rural populations, and participants who use their phones mainly for calls and texts are often better reached by SMS than any other digital channel. Excluding it means designing out a segment of your population.

A rapid review of digital engagement strategies across large cohort studies backs this up at a broader scale: SMS and email reminders were among the most commonly used design features for engaging and retaining participants across the studies reviewed, and digital reminders specifically increased medication adherence, while multiple SMS messages specifically increased clinic attendance. The two effects aren't interchangeable, which is worth remembering when deciding what a message is actually meant to achieve.

Where it fits in practice

SMS works best as part of a layered communication strategy:

• Reminders about upcoming appointments or diary entries
• Links to surveys or eConsent documents without requiring app login
• Notifications that a new task is ready in the app
• A brief follow-up after a missed data entry, along the lines of "we noticed you haven't logged today, no pressure but we're here if you need help"

Some trials use SMS as a primary channel for certain participant groups and as a backup for others. Both approaches can work. Having it available means that when the app fails, or when a participant drifts, there is still a way to reach them.

What SMS cannot do

It should not be used to collect sensitive health data directly. It is not suitable for anything requiring formatting, file attachments, or security controls. Used outside those limits, it creates more problems than it solves, and as TEXT2U shows, it shouldn't be mistaken for the intervention itself.

But within those limits, it is one of the most reliable tools in the engagement toolkit, backed by trial evidence rather than assumption. A short, well-timed message can reinforce a sense of connection and make a digital study feel more human. And that, in the end, is what keeps people involved.