Liquid and vapor can cause severe burns to the skin, eyes, and respiratory tract; corrosive and moderately toxic by ingestion; probable human carcinogen (OSHA "select carcinogen").

Dimethyl sulfate is extremely hazardous because of its lack of warning properties and delayed toxic effects. The vapor of this compound is extremely irritating to the skin, eyes, and respiratory tract, and contact with the liquid can cause very severe burns to the eyes and skin. Ingestion of dimethyl sulfate causes burns to the mouth, throat, and gastrointestinal tract. The effects of overexposure to dimethyl sulfate vapor may be delayed. After a latent period of 10 hours or more, headache and severe pain to the eyes upon exposure to light may occur, followed by cough, tightness of the chest, shortness of breath, difficulty in swallowing and speaking, vomiting, diarrhea, and painful urination. Fatal pulmonary edema may develop. Systemic effects of dimethyl sulfate include damage to the liver and kidneys.

Dimethyl sulfate is listed by IARC in Group 2A ("probable human carcinogen") and is classified as a "select carcinogen" under the criteria of the OSHA Laboratory Standard. Data indicate that dimethyl sulfate does not specifically harm unborn animals; dimethyl