Negative affectivity is a measure of anxiety associated with increased reporting of symptoms. Few studies have explored this association with respect to drug-induced symptoms in patients taking medication for a chronic disease in real life. In this cross-sectional study we examined the relationship between negative affectivity and self-reported side effects of inhaled corticosteroids in patients with asthma. We also investigated differential associations due to side effect type (subjective versus observable side effects) and treatment impact (i.e., hierarchical dosing). A total of 228 asthma patients, taking inhaled corticosteroids, completed scales measuring inhaled corticosteroid-induced side effects (Inhaled Corticosteroid Questionnaire scored: 0 = none; 100 = worst) and negative affectivity (Positive and Negative Affect Schedule scored: 10-50). Patients were grouped into low, average, and high negative affectivity groups based on published norms. Patients high in negative affectivity reported significantly greater (p < 0.001) side effects (median score 20.5 (IQR: 11.4-33.0) than the groups of patients scoring lower on this measure (low negative affectivity: 7.1 (3.1-15.6); average: 13.3 (4.9-23.3)). The relationship between negative affectivity and side effects was stronger among patients taking low (r = 0.40-0.45) rather than mid to high inhaled corticosteroid doses (r = 0.16-0.28). Asthma patients with higher negative affectivity using inhaled corticosteroids report increased medication-induced symptoms. Clinicians should be aware that aside from inhaled corticosteroid dosage, the personality of the patient is an important factor in the reporting of drug-related side effects.