A warning for you parents of young children: Mixing Fever-Reducing Drugs Is Bad for Children. Many pediatricians have advised fever-phobic parents that, instead of holding to the four-to-six-hourly dosage interval for either ibuprofen (Motrin etc.) or acetaminophen (Tylenol), parents could alternate doses of the two agents every two or three hours. Word is that this can cause additive side effects and do more harm than good.