Even though Claritin-D (loratadine; pseudoephedrine) and ZzzQuil (diphenhydramine) both contain antihistamines, their use together, on an intermittent basis, is generally considered to be safe for most individuals.
There would be a slight increase in the risk of certain side effects, like dry mouth and drowsiness since you are taking two different antihistamines, but these won't be pronounced for most individuals since Claritin is essentially non-drowsy on its own and very well tolerated overall.
In your situation, you are especially unlikely to be negatively affected by this combination since you are looking at taking the Claritin-D in the morning, and ZzzQuil at night to help you sleep (therefore, you being a little more tired than usual won't really matter in this context).
The Drugs Involved
Let's take a closer look at all the drugs involved here. Claritin-D contains two:
ZzzQuil contains one:
Loratadine is a 'second-generation' antihistamine, which is noted for its long duration of action (around 24 hours) and non-sedating effects. Other 'second-generation' antihistamines include Zyrtec (cetirizine) and Allegra (fexofenadine).
Pseudoephedrine is a nasal decongestant and the 'D' in Claritin-D. It is not an antihistamine and is safe to use with them. That is why it is so common to see antihistamine-decongestant combination products (e.g. Claritin-D, Zyrtec-D and Allegra-D etc...).
Diphenhydramine is a 'first-generation antihistamine' and is also the active ingredient in Benadryl. It is short-acting, lasting around 4 to 6 hours per dose, and causes a good deal of drowsiness in most people who take it. For this reason, it is commonly used as a sleep aid.
Loratadine With Diphenhydramine
So, when looking at a potential interaction between Claritin-D and ZzzQuil, we are really focusing in on two drugs, loratadine and diphenhydramine.
Both are antihistamines, which are also known as 'H1 blockers'. Even though all antihistamines work this way (by blocking histamine from binding to the histamine-1 receptor), they all carry different degrees of sedative effects, mostly based on whether or not they cross into the blood-brain barrier.
'First-generation' antihistamines are known for their sedative effects while newer ones, second and third-generation ones, are not (even though many do still cause some degree of sedation).
As mentioned, loratadine is a 'second-generation' antihistamine and has a favorable side effect profile in that it doesn't generally cause the sedation that older, first-generation ones do. In fact, it is even marketed as 'non-drowsy'.
Now, even though loratadine is 'non-drowsy', studies have shown that high doses of ut (over the recommended dose of 10mg per day) can cause drowsiness. Therefore, a reasonable assumption is that combining it, even within the recommended dosing, with another antihistamine (like diphenhydramine) could result in some additive side effects, like sedation.
However, since you are likely taking ZzzQuil in the evening to help you sleep, feeling a little more tired wouldn't necessarily be a bad thing.
SummaryClaritin-D and ZzzQuil both contain antihistamines, loratadine and diphenhydramine, respectively. Taking both together could increase the feeling of certain side effects, like sedation, but they are generally safe to take together once in a while.