Single-pill combination antihypertensives have mixed adherence effects

By Eleanor McDermid, Senior medwireNews Reporter

Switching to single-pill combination (SPC) antihypertensives results in marked improvements in medication adherence, except for in a subset of patients for whom the strategy may backfire, say researchers.

Tzung-Dau Wang (National Taiwan University Hospital, Taipei City) and co-workers found that patients with already high adherence to multiple single antihypertensive agents tended to become less adherent when switched to SPCs.

There were 896 patients in the study, of whom 19% had a medication possession ratio (MPR) of at least 0.8 at baseline, indicating high (80%) adherence. From the year before to the year after switching to a SPC, adherence among these patients fell by a significant 13 percentage points, from an average MPR of 95.3% to 82.3%.

By contrast, adherence among patients with low baseline adherence (MPR<0.5) increased by 43.9 percentage points when they switched to a SPC, from an average of 20.1% to 64.0%. Patients with intermediate adherence had a small but significant improvement of 7 percentage points.

“[D]espite the seemingly magic bullet effect of SPCs on improving medication adherence, this strategy is not effective and may even worsen adherence in patients with hypertension adequately adhering to their original free-combined antihypertensive regimens,” the team writes in Hypertension.

There was evidence that this was partly caused by regression to the mean, in that a similar bidirectional change in medication adherence was seen among 2577 patients who took free combinations and 831 who took SPCs of antihypertensives across the whole study period. However, the change among patients who switched to SPCs remained statistically significant after accounting for this phenomenon, among other potential confounders.

Use of SPCs has been encouraged partly due to the belief that increased complexity of medication and number of pills taken adversely affects patients’ adherence. However, Wang et al found that the positive effects of switching to a SPC declined the more pills patients were initially taking.

“It is plausible that patients treated with more medications usually have more severe clinical conditions and are more willing to follow the therapeutic regimen,” they say.

The initial MPR of 44% among patients initially taking one or two additional antihypertensive drugs rose to 71% and 72%, respectively, when they switched to a SPC. But among those taking three additional antihypertensives, MPR rose from 42% to just 57%.

The greater success of switching to SPCs among patients taking fewer antihypertensives leads the researchers to endorse “early or even initial use of SPCs to curtail the gaps between evidence and sustained implementation of antihypertensive therapy.”

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