Drugs.com recommends if you experience any of the following serious side effects, stop taking acetaminophen, aspirin, and caffeine and seek emergency medical attention or contact your doctor immediately: an allergic reaction (difficulty breathing; closing of the throat; swelling of the lips, tongue, or face; or hives); liver damage (yellowing of the skin or eyes, nausea, abdominal pain or discomfort, unusual bleeding or bruising, severe fatigue); or blood problems (easy or unusual bleeding or bruising). black, bloody, or tarry stools; nausea, vomiting, or abdominal pain; uncontrolled fever; blood in the urine or vomit; decreased hearing or ringing in the ears; seizures; or dizziness, confusion, or hallucinations.

These side effects are very rare and are not likely to occur with the proper use of acetaminophen, aspirin, and caffeine. If you experience any unusual side effects, stop taking this medicine and seek the advice of your doctor.
If you experience any of the following less serious side effects, notify your doctor: heartburn or indigestion; or nervousness, anxiety, or difficulty sleeping.