The combination erythromycin 2% and metronidazole 2% incompatible with Unguentum aquosum in the aqua??

The alkaline pH will not be the cause but rather the presence of after lauryl sulfate. After all, I note that   in a foreign formulary an analogue cream (metronidazole 1%) is prepared with a non ionogenic emulsifier being   PEG-400 monostearate.  

I should also point out that the alkaline pH of this cream is detrimental to the stability of erythromycin and that you should use 100 g of cream 12.0   g   0.5% citric acid solution. Check the composition of the erythromycin preparations described in the "bible". The book by which a future pharmacist goes to sleep