Ondansetron and Lansoprazole Do Not Significantly Alter Gabapentin or Buspirone Absorption
No dose adjustments are required for gabapentin or buspirone when co-administered with ondansetron (Zofran) or lansoprazole, as these medications do not have clinically significant drug-drug interactions affecting absorption.
Ondansetron (Zofran) Drug Interactions
Ondansetron does not induce or inhibit the cytochrome P-450 drug-metabolizing enzyme system 1. The FDA label explicitly states that ondansetron does not affect the pharmacokinetics of other medications through metabolic pathways 1. Research confirms ondansetron is rapidly absorbed with approximately 60% bioavailability, and this absorption is not influenced by coadministration of antacids 2.
For gabapentin: No interaction exists. Ondansetron works as a 5-HT3 receptor antagonist with purely antiemetic effects and does not affect gastric motility or drug absorption 1. Gabapentin absorption occurs via the L-amino acid transporter system in the small intestine, which is unaffected by ondansetron.
For buspirone: The FDA label for buspirone does not list ondansetron as an interacting medication 3. Buspirone is metabolized primarily by CYP3A4, and ondansetron does not inhibit or induce this pathway 1.
Lansoprazole Drug Interactions
Lansoprazole is extensively metabolized by CYP3A4 and CYP2C18 4. Studies show lansoprazole does not modify the pharmacokinetics of warfarin, prednisone, theophylline, diazepam, phenytoin, or oral contraceptives, suggesting minimal risk of clinically significant interactions 4.
For gabapentin: Research demonstrates that magnesium-containing antacids significantly reduce gabapentin absorption (33% decrease in Cmax, 43% decrease in AUC, 32% decrease in bioavailability), but proton pump inhibitors like omeprazole (and by extension lansoprazole) do not affect gabapentin pharmacokinetics 5. The study showed no significant differences in plasma disposition parameters between gabapentin alone versus gabapentin with omeprazole 5. This is because PPIs do not chelate gabapentin or alter intestinal pH in a way that affects the L-amino acid transporter.
For buspirone: Lansoprazole is not listed among the CYP3A4 inhibitors or inducers that affect buspirone metabolism 3. The FDA label specifically identifies potent CYP3A4 inhibitors (erythromycin, itraconazole, nefazodone, diltiazem, verapamil) and inducers (rifampin, dexamethasone, phenytoin, carbamazepine) that require buspirone dose adjustment—lansoprazole is not among them 3.
Critical Caveat: Magnesium-Containing Antacids
If the patient is taking magnesium oxide or other magnesium/aluminum-containing antacids concurrently with lansoprazole, separate gabapentin administration by at least 2 hours from the antacid. Research shows magnesium oxide reduces gabapentin bioavailability by 32-39% through decreased intestinal absorption 5. Lansoprazole bioavailability itself is decreased 27% by food and slightly by concomitant antacids 6, but this does not translate to effects on other medications.
Practical Prescribing Guidance
- Continue gabapentin at current dose when adding ondansetron or lansoprazole
- Continue buspirone at current dose when adding ondansetron or lansoprazole
- Monitor for therapeutic efficacy of gabapentin (pain control, seizure control) and buspirone (anxiety control) as you would routinely
- Avoid magnesium/aluminum antacids within 2 hours of gabapentin if patient requires additional acid suppression beyond lansoprazole
- No laboratory monitoring required for these specific drug combinations
The evidence consistently shows these are safe combinations without absorption interference requiring dose modification 1, 5, 2, 4.