If i take 5mg of zyrtec (cetirizine) at night, is it still working come morning and afternoon, or should i take the full 10mg?

Most pollen from : trees are released at night between 3 AM to 6 AM. So your Zyrtec (cetirizine) should work fine for the time when you need it the most! However if you're in a hyper allergenic environment, you may need the full dose. So if you feel that all you need is the evening and early morning doses the 5 mg may be enough. Let your body be your barometer!

May work: If your symptoms are controlled....then you are ok....in general....adults doses are 10 mg