Is it safe to take niquil with claritin (loratadine)?

Probably ok to do: Claritin (loratadine) is a once-a-day medication, and does wear off some by night time. If nyquil, with its own antihistamine, is taken at night time, there should not be a big problem. Nyquil is usually used for just a few days during cold or flu symptoms. A doctor can evaluate prolonged or bothersome symptoms, to see if there something serious going on.

Yes: Claritin is a non-sedating antihistamine that does so by not crossing into the brain. The Nyquil has an antihistamine that does cross into the brain where it interferes with the brain messengers. This causing drowsiness. It is that side effect that makes it useful to some. I see no reason to avoid it. You can get a similar effect safely with diphenhydramine.