Anesthesia is a medication that can be used to induce sleep, pain relief, and unconsciousness. Anesthesia is a medication that can be used to induce sleep, pain relief, and unconsciousness. It is given to patients before surgery or other medical procedures. Side effects of anesthesia include nausea and vomiting, headache, drowsiness, dizziness, breathing problems (such as hypoxia), confusion or amnesia after surgery or during the recovery process.