It depends on which Robitussin you have: plain Robitussin (guaifenesin and dextromethorphan) is commonly taken with Tylenol, but "Severe" or "Cold + Flu" Robitussin formulas already contain acetaminophen — the same drug as Tylenol — so combining those would double-dose you and risk liver damage. Always read the label and ask a pharmacist.
"Robitussin" is a brand covering several different formulas, so the answer hinges on the active ingredients. The classic products (e.g. Robitussin Cough Plus Chest Congestion DM) contain only guaifenesin, an expectorant that thins mucus, plus dextromethorphan, a cough suppressant; neither is expected to interact dangerously with acetaminophen (Tylenol), and their FDA/DailyMed labels list no acetaminophen and no liver warning. However, the multi-symptom Robitussin Severe Multi-Symptom Cough Cold + Flu product contains 650 mg of acetaminophen per dose and carries the FDA-label warning "Do not use with any other drug containing acetaminophen (prescription or nonprescription)," because stacking it on top of Tylenol can push you over the safe acetaminophen limit and cause severe liver damage. So the combination is fine only when your Robitussin does NOT already contain acetaminophen.
Check the Drug Facts "active ingredients" box on your Robitussin before adding Tylenol. If acetaminophen is listed (common in "Severe," "Cold," "Flu" or "Multi-Symptom" versions), do NOT also take Tylenol — that doubles the dose. If your Robitussin is only guaifenesin and/or dextromethorphan, taking it with Tylenol is generally fine, but stay within acetaminophen limits: the NHS caps healthy adults at 4,000 mg per day (no more than eight 500 mg tablets in 24 hours), and US OTC labels often use a more conservative 3,000 mg/day limit — leave at least 4 hours between doses and do not use for more than about 3 days without medical advice. Avoid heavy or daily alcohol use, since the Robitussin Severe label itself warns that taking acetaminophen with 3 or more alcoholic drinks every day raises the risk of liver damage. People with liver disease, heavy alcohol use, or who take other cold/flu or prescription products containing acetaminophen should ask a doctor or pharmacist first. Seek urgent medical help (call Poison Control or go to the ER) for signs of acetaminophen overdose such as nausea, vomiting, loss of appetite, or upper-right abdominal pain, even if you feel okay at first.
This is general reference, not medical advice, and not a guarantee of safety. Interactions depend on your doses, health conditions, and other medicines. Always confirm with your pharmacist or doctor before combining products, and follow the dosing on each label.