Chills are a recognized, though less common, side effect of Sudafed (pseudoephedrine), often accompanied by nervousness, tremors, or a rapid heart rate due to its stimulant effects. While mild, temporary chills may occur, they can also indicate a severe adverse reaction or allergic response. Seek immediate medical attention if chills are accompanied by fever, chest pain, or severe dizziness. Key Information regarding Sudafed and Chills: Common Side Effects: Along with chills, you may experience nervousness, restlessness, insomnia, dizziness, and tremors. When to Stop Taking: Stop using pseudoephedrine and call your doctor if you experience fever, chills, body aches, severe dizziness, or a fast/pounding heartbeat. Serious Warnings: Sudafed can increase blood pressure and heart rate, which may lead to serious cardiovascular issues. Why it Happens: As a sympathomimetic amine, it acts as a stimulant, affecting the central nervous system and causing peripheral vascular constriction, which can feel like sudden coldness or chills, according to MedicineNet. It is recommended to use pseudoephedrine for short-term relief (usually less than 7 days) and to consult a doctor if side effects are bothersome.