Bioavailability of MTX

At higher doses, decreased bioavailability may be even more pronounced. Thus, patients may appear to respond better to parenteral therapy, when the actual explanation is that more drug is reaching the circulation. As the variation in the confidence intervals for the drop-off in f extend to 27 percent, selected patients may have an even greater difference in f between parenteral and oral dosing. Since f may drop off by a mean of 13.5 percent as the dose of MTX is increased from a starting dose of 7.5 mg to the typical maintenance weekly doses employed in RA , a patient may do better clinically when switched from oral to intramuscular or subcutaneous administration of the same dose of the drug. Bioavailability is defined as the ratio of oral drug absorption divided by intravenous drug absorption. Oral MTX is variably absorbed in the dosage range used to treat RA.