BACKGROUND: Lipid-lowering drugs are extensively used in primary care to reduce the risk of cardiovascular disease (CVD). Apart from high total cholesterol (TC), several other clinical-chemical variables are associated with the risk of CVD. Magnesium-pyridoxal-5'-phosphate-glutamate (MPPG) has been found to have a positive influence on TC levels and other clinical-chemical values in some selected populations. OBJECTIVE: To assess, in a general practice (GP) setting, the efficacy and clinical effectiveness of MPPG in the treatment of clinical-chemical risk factors for CVD. DESIGN: Randomised double-blind, placebo-controlled, clinical trial, lasting 12 months. PATIENTS: Adults (25-66 years) in an average Dutch village population with serum cholesterol levels between 7.0 mmol/l and 9.9 mmol/l. INTERVENTION: Subjects were assigned at random to treatment with MPPG (3 x 150 mg daily) or placebo. Clinical-chemical parameters were assessed at 1, 3, 6, 9 and 12 months (t1, t2, t3, t4, t5). Efficacy was measured at t2. Long-term effect (clinical effectiveness) was measured by combining the results at t2, t3, t4 and t5 (t2-5). OUTCOME MEASURES: TC (primary), low density lipoprotein-cholesterol (LDL-C), high density lipoprotein-cholesterol (HDL-C), triglycerides, apolipoprotein-A1 (Apo-A1), apolipoprotein-B100 (Apo-B), fibrinogen and lipoprotein a [Lp(a), secondary]. RESULTS: No statistically significant differences in the efficacy and effectiveness of TC were found between the MPPG group and the placebo group. The same was demonstrated for the other clinical-chemical values, except for LDL-C (effectiveness, P = 0.04). CONCLUSIONS: Efficacy and effectiveness of MPPG are too poor to be of relevance for application as a lipid-lowering drug in GP.