Taking melatonin can lead to various negative side effects including dizziness, headaches, irritability, nausea, anxiety, and stomach cramps. When used habitually, melatonin supplements may also interfere with the body’s natural melatonin production. As a result, it may be increasingly difficult to maintain consistent sleep and wake times. In severe cases, it could cause shortness of breath, chest pain, and/or significant changes in blood pressure. If this happens, it is vital to seek medical attention immediately.