Dexamfetamine (Amfexa) pharmacokinetics with magnesium oxide antacids
No controlled pharmacokinetic study was identified showing that magnesium oxide taken immediately before dexamfetamine (Amfexa) changes plasma dexamfetamine concentrations or dexamfetamine elimination half-life.
Dexamfetamine exposure and elimination depend on urinary pH because urinary excretion of amfetamine is pH-dependent. Amfexa SmPC (Medice/UK emc)
Expected effect on plasma drug levels
Dexamfetamine from Amfexa is rapidly absorbed after oral administration. Amfexa SmPC (Medice/UK emc)
Magnesium oxide is an oral antacid, and antacids can affect drug absorption by altering gastrointestinal conditions (eg, gastric pH and dissolution/release behavior). [1]
Because no dexamfetamine–magnesium oxide coadministration data were found, the direction and magnitude of any change in dexamfetamine Cmax/AUC with immediate dosing remain unknown.
Expected effect on elimination half-life
Amfexa prescribing information reports an average dexamfetamine plasma elimination half-life of 10.2 hours. Amfexa SmPC (Medice/UK emc)
Dexamfetamine elimination is pH-dependent: at low urine pH, a much larger fraction is eliminated unchanged (reported as ~80% within 24 hours), whereas at alkaline urine pH, only ~2–3% is eliminated unchanged (implying slower elimination). Amfexa SmPC (Medice/UK emc)
No evidence was identified that magnesium oxide taken immediately before Amfexa produces a reproducible change in urinary pH sufficient to measurably alter terminal half-life of dexamfetamine.
Practical implication for timing
If avoidance of an absorption-related interaction is desired, separation of dosing is commonly used for drugs affected by antacids because antacids can alter gastrointestinal absorption conditions. [1]
However, for magnesium oxide specifically with dexamfetamine, the quantified effect on plasma levels and half-life is not established from available PK data.