Methods are disclosed for preventing toxic drug-drug interactions during combination cancer therapy with a drug that is an anti-ErbB3 agent, such as an anti-ErbB3 antibody, together with a drug that is a tyrosine kinase inhibitor and/or a drug that binds to alpha- 1 acid glycoprotein (e.g., erlotinib). Health care practitioners obtaining any one of the drugs are warned that when co-administering the drug that is an anti-ErbB3 agent with either or both of a drug that is a tyrosine kinase inhibitor and a drug that binds to alpha- 1 acid glycoprotein, at least one of the co-administered drugs should be administered using a reduced dosage to prevent toxicity. In a reduced dosage, the amount of drug administered per unit time is reduced as compared to a dose that would be administered if the drug was administered as monotherapy. The reduced dosage can be, for example, a reduced drug dose or a reduced drug dosing frequency, or both. Compositions useful in practicing the disclosed methods are also provided.