BuSpar (Buspirone) serious side effects. Seek immediate medical attention if any of these rare but serious side effects occur: easy bleeding/bruising, shortness of breath, chest pain, fast/irregular heartbeat. A very serious allergic reaction to this drug is unlikely, but seek immediate medical attention if it occurs. Symptoms of a serious allergic reaction may include: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing.