Background:
     
     As clinical practice guidelines represent the most important evidence-based decision support tool, several strategies have been applied to improve their implementation into the primary health care system. This study aimed to evaluate the effect of intervention methods on the guideline adherence of primary care providers (PCPs).
   

Methods:
     
     The studies selected through a systematic search in Medline and Embase were categorised according to intervention schemes and outcome indicator categories. Harvest plots and forest plots were applied to integrate results.
   

Results:
     
     The 36 studies covered six intervention schemes, with single interventions being the most effective and distribution of materials the least. The harvest plot displayed 27 groups having no effect, 14 a moderate and 21 a strong effect on the outcome indicators in the categories of knowledge transfer, diagnostic behaviour, prescription, counselling and patient-level results. The forest plot revealed a moderate overall effect size of 0.22 [0.15, 0.29] where single interventions were more effective (0.27 [0.17, 0.38]) than multifaceted interventions (0.13 [0.06, 0.19]).
   

Discussion:
     
     Guideline implementation strategies are heterogeneous. Reducing the complexity of strategies and tailoring to the local conditions and PCPs' needs may improve implementation and clinical practice.