Caution is recommended when prescribing magnesium supplements. Raltegravir binds to divalent cations such as magnesium and forms a complex at the level of the gastro-intestinal tract which results in less raltegravir being absorbed. Administration of raltegravir 400 mg twice daily should be separated by at least 4 hours. Importantly, drug-drug interaction studies with antacids containing divalent cations have shown a more pronounced reduction in raltegravir Cmin when raltegravir was administered once daily compared to a twice daily regimen. A similar effect for magnesium supplements cannot be excluded, therefore, twice daily administration of raltegravir should be preferred.