Dragon Effects are a series of things known to cause symptoms to dragons, but many of the effects do not have a good effect on dragons.
Most dragons get eel pox after eating an eel. Some of the symptoms are an infinite number of uncontrollable shots, bubbly and red-tinted vision, and possibly fever. This is the reason why dragons tend to avoid eels, all except the Typhoomerang and the Death Song.
When dragons are given dragon nip, they temporarily become docile in nature. It is known to be effective on almost every dragon, except Whispering Deaths.
When dragons are near Blue Oleander, they start sneezing, having defective firepower, possible fever, and weakness.
This effect has only been seen in Stoker Class dragons so far. They get the inability to produce fire, weakness, and turn noticably pale. Hookfang was cured from this with the venom of the Fireworm Queen.
When dragons are around this root, they get aggressive behavior, wanting to have the root immediatly.
Dragon Hunters have melted it down and coated their arrows with it. When shot, a dragon's senses are overloaded and they can't fly. They become sluggish and can't move on their own.
Dragon root is ineffective on Gronckles because of their stone diet. It may possibly be inaffective on Hotpurples as well.
When dragons get toothaches, or have a bad tooth, they become aggressive in behavior. They won't eat or chew because of the tooth causing too much pain.
Monstrous Nightmares have been known to spontaneously set themselves on fire.
However, if you remove the tooth without getting your hand bitten off, the dragon will be fine afterwards.