Antihistamines are like a double-edged sword. They can cut both ways. They dry up the secretions. They slow down or paralyze the cilia (microscopic hair-like projections on the mucosal cells), which propel the secretions. This causes the thickened mucus secretions to stay inside the sinuses and not drain outward. Unless you are drowning in nasal secretions and need to slow it down temporarily, extended use of anti-histamines will often swap you a runny nose for a headache.