instruction: get me the number of patients on additive type drug prescription who have diagnoses icd9 code 45981.
response: SELECT COUNT(DISTINCT demographic.subject_id) FROM demographic INNER JOIN diagnoses ON demographic.hadm_id = diagnoses.hadm_id INNER JOIN prescriptions ON demographic.hadm_id = prescriptions.hadm_id WHERE diagnoses.icd9_code = "45981" AND prescriptions.drug_type = "ADDITIVE"