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Over the counter medications are not safe for cats so you cannot give any antihistamines, etc. You CAN use a couple of drops of infant saline nasal spray to help ease the conjestion. If it is really bad, you can use Afrin nasal spray. Only use 1 drop in one nostril. Then tomorrow you can put one more drop in the other nostril.
You can also try running a humidifier or adding moisture to the air with a pot of simmering water.
Let me know if you still have questions.