Black seed is generally considered to be safe and well-tolerated. The most commonly reported side effects are gastrointestinal in nature, including diarrhea, nausea, vomiting, and heartburn.[1][2]
There have been several case reports of allergic skin reactions following the topical or oral use of black seed, ranging from mild dermatitis and hives to cases of Stevens-Johnson syndrome or drug reaction with eosinophilia and systemic symptoms (DRESS) syndrome, both of which are severe hypersensitivity reactions that can result in extensive blistering/rashes, fever, and potential damage to internal organs.[3][4]