Gastric pH–Mediated Effects on Immediate-Release Dextroamphetamine (Amfexa)
Gastric acidity–modifying agents can change gastrointestinal absorption of immediate-release dextroamphetamine by altering gastric pH. Gastrointestinal acidifying agents reduce absorption and blood levels. Gastrointestinal alkalinizing agents increase absorption and blood levels, thereby potentiating clinical effects.
Mechanism of Action on Absorption and Pharmacokinetics
Dextroamphetamine is a weak base, so gastrointestinal alkalinization increases the fraction in the non-ionized form available for absorption. (pi-pil-repository.sahpra.org.za)
Gastrointestinal acidifying agents decrease dextroamphetamine absorption, lowering systemic exposure. (pi-pil-repository.sahpra.org.za)
Gastrointestinal alkalinizing agents increase absorption, increasing systemic exposure. (pi-pil-repository.sahpra.org.za)
Antacids (Gastrointestinal Alkalinizing Agents)
Gastrointestinal alkalinizing medicines (including antacids such as sodium bicarbonate) increase absorption of amfetamines. (pi-pil-repository.sahpra.org.za)
Co-administration of immediate-release amphetamine products with gastrointestinal alkalinizing agents “should be avoided” because these agents increase absorption and can increase blood levels. [1]
H₂-Receptor Antagonists and Proton-Pump Inhibitors
Gastric acid–suppressing therapy can function as a gastric pH–elevating exposure modifier. [2]
Amfexa labeling classifies the relevant interaction category as “gastrointestinal acidifying” versus “gastrointestinal alkalinizing” agents, with alkalinizing agents increasing absorption and blood levels. (pi-pil-repository.sahpra.org.za)
Amfexa-specific labeling does not provide a quantified absorption or exposure change for H₂-receptor antagonists or proton-pump inhibitors, but the pharmacologic interaction direction is consistent with gastric pH elevation. (pi-pil-repository.sahpra.org.za)
Monotherapy Versus Combination With Acid-Reducing Agents
When acid-reducing therapy increases gastric pH (alkalinizing effect), systemic exposure to immediate-release dextroamphetamine increases. (pi-pil-repository.sahpra.org.za)
When acid-reducing therapy decreases gastrointestinal acidification (alkalinizing effect), the stimulant actions of dextroamphetamine are expected to be potentiated based on labeling statements about increased blood levels. (pi-pil-repository.sahpra.org.za)
When gastrointestinal acidifying agents are co-administered, systemic exposure and efficacy are expected to decrease based on labeling statements. (pi-pil-repository.sahpra.org.za)
Clinical Efficacy Impact
Gastrointestinal acidifying medicines lower absorption and lower blood levels of amfetamines, with reduced efficacy stated in labeling. (pi-pil-repository.sahpra.org.za)
Gastrointestinal alkalinizing medicines increase absorption and blood levels of amfetamines, thereby potentiating the actions of amfetamines. (pi-pil-repository.sahpra.org.za)
Quantified changes in ADHD or narcolepsy symptom control with specific antacid/H₂-receptor antagonist/proton-pump inhibitor combinations are not provided in the available Amfexa interaction text. (pi-pil-repository.sahpra.org.za)
Important Pharmacokinetic Nuance: Urinary pH Effects Also Modify Exposure Duration
Amfexa labeling distinguishes gastrointestinal effects from urinary effects. (pi-pil-repository.sahpra.org.za)
Urinary acidifying medicines increase the concentration of the ionized species, increasing urinary excretion, with both lower blood levels and reduced efficacy stated in labeling. (pi-pil-repository.sahpra.org.za)
Urinary alkalinizing medicines decrease urinary excretion by increasing the non-ionized fraction, increasing blood levels and potentiating actions. (pi-pil-repository.sahpra.org.za)
Experimental data show amphetamine elimination kinetics differ under urine acidification versus alkalinization conditions. [3]
Common Pitfalls to Avoid
Using gastrointestinal alkalinizing agents (including antacids) concurrently with immediate-release amphetamine products is a listed interaction risk, with guidance to avoid co-administration for alkalinizing agents. [1]
Focusing only on gastrointestinal pH can miss the parallel and clinically relevant urinary pH interaction, which can prolong exposure through reduced urinary excretion when urine is alkalinized. (pi-pil-repository.sahpra.org.za)