Hi Matty,
I've also heard that if you have candida (yeast) overgrowth, it's best to get that under control before starting LDN. That said, I don't know what happens specifically if you take LDN with the overgrowth -- it might just interfere with the LDN, so there is no effect to the medication. I'm also unfamiliar with allergies interfering with LDN. The side-effect profile is low, and I haven't heard about any adverse reactions from folks who have taken LDN, so perhaps it isn't an issue. Some people have reported things getting worse before they get better, but in general, I haven't heard of anyone having an allergic reaction to LDN or any bad side effects aside from vivid dreams.