Chiropractic care is frequently sought by patients for relief from neck pain; however, adverse reactions related to its primary modes of treatment have not been well examined. A total of 336 patients with neck pain presenting to four southern California healthcare clinics were randomised in a balanced 2×2×2 factorial design to manipulation with or without heat, and with or without electrical muscle stimulation (EMS); and mobilisation with or without heat and with or without EMS. Discomfort or unpleasant reactions from chiropractic care were self-assessed at 2 weeks after the randomisation/baseline visit. Of the 280 participants (83%) who responded, 85 (30.4%) had 212 adverse symptoms as a result of chiropractic care. Increased neck pain or stiffness was the most common symptom, reported by 25% of the participants. Less common were headache and radiating pain. Patients randomised to manipulation were more likely than those randomised to mobilisation to have an adverse symptom occurring within 24 h of treatment.