Can my 18 year old daughter, 100 lbs, take claritin (loratadine) and dayquil cold and flu?

Better not: Both Claritin (loratadine) and dayquil cold and flu contain anti-histamine. Do not take both at the same time. Look at the label carefully, consult your pharmacist when you purchase the medicines.

DayQuil: Yes. Dayquil cold and flu contains Acetaminophen or tylenol (acetaminophen) which helps with pain and fever, phenylephrine which helps with congestion and dextromethorphan which is a cough suppressant. As none of these ingredients is an antihistamine, she can take Claritin safely, which is considered a "non-sedating" antihistamine. A little caution though as sedation can be a side effect of either of these.

Generally yes: History, allergies, current meds, diseases, special needs- issues are relevant to any and all meds.