Can anyone tell me why combining dxm and allegra is bad

[Joewenston]

Allegra D (12 hours) is the allergy medicine i take, but in a faq i read i beleive by William White he said taking Allegra and dxm is bad and fatal? It was suggested wait 2 weeks but i have taken it in the morning and dxm later that night and been fine several tiems. But today i took my allegra around 7:40am and i just popped 9 robo gels and i stopped cause i forgot i took my medicine earlier..

am i trippin or will i be alright?

 

[Solipsis]

Let me look into it, but off the top of my head it may be that metabolic enzymes are inhibited competitively giving overdose risks...

edit: hmm that's not it, I am reading that adverse anticholinergic effects may occur.
You're right to be confused IMO, caution is advised by FAQ(s) but reasons withheld.

edit: whoops I seem to've been right the first time, quoting our own Jasoncrest:

There are interactions if the antihistamine you use is metabolized by CYP2D6 (like a lot of antihistamines).
Try to find on google if your antihistamine is metabolized by CYP2D6.

well...

Fexofenadine in turn is not metabolized by cytochrome
P450, and over 95% of the molecule is recovered in
urine and stools [34].

http://www.jiaci.org/issues/vol16s1/5.pdf

Allegra is fexofenadine.
CYP2D6 and CYP3A4 (the latter metabolizes terfenadine, another drug and a prodrug of fexofenadine) are isoforms of cytochrome P450. Apparently what fexofenadine IS a substrate for is PgP. Hmm doesn't ring a bell, I've been out of college too long?

So I think you're okay and it explains why you had no problems. Be safe.

 

[Jak3]

You are %100 fine, it all depends on the amount of DXM ingested though. Plenty of people pop benadryl, allegra, and several other anti-histamines to relieve themselves from the horrible itch. Though I wouldnt recommend taking it when using large doses of DXM because thats when it becomes fatal.

 

[psood0nym]

White is occasionally full of shit (like all those claims about Olney's lesions he later recused himself of). The only reason I can see looking around online is that fexofenadine is "minimally metabolized by CYP enzymes". DXM's metabolism is most strongly associated with cyp2d6, and so using Allegra with it might, and only "might," lengthen the trip slightly or cause a slightly higher ratio of DXM to DXO. I see no reason for concern, especially if it's just your normal Allegra dose. Enjoy your trip.

 

[Joewenston]

Ah thanks everyone really i appreciate it!
Feel much much better now.

 

[flowerb0x]

The interaction was with terfenadine specifically. In the FAQ it was written not to combine DXM with *any* prescription antihistamine just in case the problem extended to the others, which were new at the time. Terfenadine ended up having so many drug interactions with other drugs that it was pulled from the market everywhere and is no longer used. The Wikipedia page for it describes the problem specifically; it can occur with any drug also metabolized by CYP3A4, which DXM (and DXO) are.

The FAQ needs to be updated on this point, but the current version (4.0) was written in March 1997 and the FDA did not recommend against prescribing terfenadine until January 1997; it was not removed from the US market until later that year. From Section 4.16: "Incidentally, this probably applies to other non-drowsy antihistamines, such as ClaritinTM and HisminalTM as well; avoid combining them with DXM." I.e. William White was hedging his bets until more information came in and then the FAQ never got updated when it did.