Can you mix robitussin and Claritin?

Yes. Drink plenty of fluids. Use Tylenol, aspirin or an NSAID for fever or headache. This applies to Claritin (Loratadine) and not Claritin D (Loratadine with Pseudoephedrine)

For Claritin D Yes. These medications each contain two drugs that treat two different conditions. Mucinex DM is an expectorant/cough suppressant. Claritin D is an antihistamine decongestant. Avoid Claritin D if you have high blood pressure or heart problems. If you develop a fever, take Tylenol (acetaminophen) or Motrin (ibuprofen) with any of these.

Source: http://www.bjchealth.org/?id=5488&sid=3