Background and aims
There is evidence that taking low dose aspirin post myocardial infarction (MI) causes a reduction in both overall mortality and reinfarction rates. However, this depends on patients adhering to a life-long daily regime where taking less than 95% is predictive of a higher risk of MI (Glynn et al., 1994). Patients' beliefs about their treatment influence how medicines are taken (Horne and Weinman in press). This study aims to investigate the role of patients' beliefs about medicines in adherence to aspirin during the first year following MI.

Method
A prospective, longitudinal study of 132 first time MI patients at four time-points (in hospital, 3, 6 and 12 months post MI) to investigate patients' representations of aspirin and other cardiac medicines (assessed using the standardised 'Beliefs about Medicines Questionnaire - Specific', Horne et al., 1999). Adherence was measured using a composite score of patients' self-reported adherence ('Reported Adherence to Medicines Scale'; Horne et al., 1999) plus a measure of prescription redemption from the GP or Pharmacy (available for aspirin in 50% of cases). High adherence was categorised as above 75% on both self-report and percentage supply measures. Differences in composite scores between aspirin and other cardiac medicines were explored using Chi-square test. Paired sample t-tests were used to investigate changes in beliefs about medicines over time and independent sample t-tests to assess differences in medication beliefs between high and low adherers.

Results
Adherence scores for aspirin were significantly lower than for other cardiac medicines at each follow up phase (3 months, Chi=5.5, p< 0.05; 6 months, Chi=11.8, p<0.01; 1 year, Chi=14.6, p<0.01) and decreased significantly between 6 months and 1 year (Chi=4.7, p< 0.05), whereas adherence to other cardiac medicines remained stable over time. The perceived need ('necessity' scale scores) for aspirin at 3 months was significantly lower than for other cardiac medicines (t=-1.8, df=64, p<0.05) which was predictive of low aspirin adherence at 6 months (t=-2.3, df=64, p<0.05). However, the perceived need for aspirin increased significantly at 6 months (t=-2.1, df=78, p<0.05) and remained high at 1 year post MI. Patients' representations of the risks ('concerns' scale scores) of taking aspirin were significantly lower than for other cardiac medicines at each time-point (3 months, t=-6.3, df=64, p<0.01; 6 months, t=-3.0, df=84, p<0.01; 1 year, t=-2.6, df=90, p<0.01). Strong concerns about aspirin at 3 months and 6 months (t=2.6, df=64, p<0.01) were predictive of poor adherence to aspirin at 1 year.

Conclusions
Following a first MI, patients may not be convinced of the necessity of taking low dose aspirin and perceive them as less important than other cardiac medicines leading to lower adherence. Misplaced concerns about aspirin (although infrequent) was another cause of sub- optimal secondary prevention of MI and death in this high-risk population.

References

1. Glynn RJ, Burning JE, Manson JE, LaMotte F & Hennekens CH. Adherence to aspirin in the prevention of MI. Arch Intern Med 1994; 154:2649-2657.
2. Horne R & Weinman J (in press). Patients beliefs about prescribed medicines and their role in adherence to treatment in chronic physical illness. Journal Psychosomatic Research.
3. Horne R, Weinman J, Hankins M. The beliefs about medicines questionnaire: the development and evaluation of anew method for assessing the cognitive representation of medication. Psychol & Health 1999; 14 (1): 1-24.