Hi saxmar,
Thanks for taking the time to respond. He's on Biaxin 500mg 2xday, Placquenil 200mg 2xday, and Rifampin 300mg 2xday. He's to take the Biaxin for 7 days, then after 7 days add the Placquenil, then after 7 more days he's to add the Rifampin, so that by the end of 21 days he's on all three meds. We've been warned he'll probably Herx badly on this combo.
If I understand the literature correctly, (which I may not) if Rifampin is added to someone already on Biaxin it reduces the effectiveness of the Biaxin by roughly half. Conversely, if Biaxin is added to someone already on Rifampin, it can (up to) double the effectiveness/dosage/toxicity of the Rifampin.
"...the addition of rifabutin in patients stabilized on clarithromycin therapy slowly decreased the clarithromycin area under the plasma concentration-time curve (AUC) and C(max) up to an average of 44% and 41%, respectively, at the end of 4 weeks of combination therapy. In patients stabilized on rifabutin therapy, the addition of clarithromycin significantly increased rifabutin AUC and C(max) after the first dose. After 4 weeks, average increases of 99% and 69%, respectively, were reported.Some experts recommend that this combination be avoided since it may result in decreased efficacy of the macrolide and increased rifamycin toxicity (e.g, neutropenia, uveitis)." http://www.drugs.com/drug-interactions/biaxin_d00097_rifampin_d00047.html
So if I understand correctly, putting him on Biaxin then adding Rifampin 21 days later will decrease the efficacy of the Biaxin by up to half. Perhaps the doctor has adjusted dosages to account for this. Perhaps this occurs only a small percentage of the time. For sure I'll have to talk to the doctor about this. I was just wondering if anyone else had any knowledge/experience with this or if it's a non issue. I'm GUESSING it's a non issue because his doctor is outstanding, and always leads us by the hand for warning us of drug interactions, food interactions with drugs, side effects, etc. Like you said, saxmar, no one reacts to the drugs to same and you're right - I guess just keep an eye on him. Any comments very appreciated.