Hi & Welcome. Avoid taking these medications together. Nyquil contains Acetaminophen, Dextromethorphan, & Doxylamine as its important active ingredients. When taken together Citalopram may interact with Dextrometharphan of Nyquil to increase the risk of a rare but serious condition called the Serotonin syndrome which is characterized by confusion, hallucination, seizure, extreme changes in BP, increased heart rate, fever, excessive sweating, shivering or shaking, blurred vision, muscle spasm or stiffness, tremor, in coordination, stomach cramp, nausea, vomiting, and diarrhea. Severe cases may result in coma and even death. Consult your treating doctor for an alternate effective but a safer medication to treat seasonal allergy like Phenylephrine nasal drops or steroidal nasal sprays along with Paracetamol. Besides. you also need to have to certain precautions to avoid seasonal allergens like 1. Use an air conditioner with a HEPA filter to cool your home in summer rather than ceiling fans. 2. Try to stay indoors when pollen counts are high. At times of year when your allergic symptoms are active 3. Keep your windows shut and limit your time outdoors 4. Consider wearing a dust mask when you’re outside, especially on windy days 5. Avoid cigarette smoke, which can aggravate your symptoms. Hope the advice given above would be useful. Best of your health. A feedback is appreciated.