Vanillin can trigger migraine headaches in a small fraction of the people who experience migraines.
Some people have allergic reactions to vanilla. They may be allergic to synthetically produced vanilla but not to natural vanilla, or the other way around, or to both.
Vanilla orchid plants can trigger contact dermatitis, especially among people working in the vanilla trade if they come into contact with the plant's sap.  An allergic contact dermatitis called vanillism produces swelling and redness, and sometimes other symptoms. The sap of most species of vanilla orchid which exudes from cut stems or where beans are harvested can cause moderate to severe dermatitis if it comes in contact with bare skin. The sap of vanilla orchids contains calcium oxalate crystals, which are thought to be the main causative agent of contact dermatitis in vanilla plantation workers.
A pseudophytodermatitis called vanilla lichen can be caused by tiny mites.