Here is a brief primer on antihistamines and nasal sprays. All this information can be retrived from the FDA, multiple research studies, The Medical Letter and the PI’s.
First off there are no studies that show increases or decreases in athletic performance in any of the antihistamines. That is ancedotal information, at best. There are studies that show significant CNS impairment and significant sedation with many antihistamines, mainly first generation antihistamines.
Claritin, Clarinex and Allegra do not impair CNS function meaning they will not make you drowsy. Allegra does not cross the blood-brain barrier. Claritin, and Clarinex, which is the active metabolite of Claritin, can cause anti-cholinergic effects such as drying of the mouth and also cause sedation and impairment if you take it at higher than recommended doses. Both are generally thought of as less efficacious than Allergra and Zyrtec. Allegra does not cause anti-cholinergic effects. Zyrtec, which was derived from Atarax which is a very powerful and very sedative antihistamie, can cause sedation even at normal doses. Personally, I’d stay away from Zyrtec before a race. Most allergists will give a slight nod to Zyrtec over Allegra for efficacy but you need to balance that out with any sedation/impairment issues you may have.
Claritin is available over the counter without a prescription. It’s compound name is Loratadine and there are several brand name and generic formulations in the pharmacy.
Benedryl is probably more effective than Allegra, Claritin, Clarinex and Zyrtec but can cause sedation and impairment of the CNS, as do all first generation antihistamines. These will be significantly less costly than claritin and state that they cause sedation on the label.
Nasal sprays are generally more effective than antihistamines but take longer to work - up to 1 week is the general rule of thumb.
As a side note, if one should choose to dope one can use a certain nasal spray to hide certain banned substances.
If you take any of the D formulations, you might notice even more drying and other side effects such as a increased feeling of restlessnes or sleeplessness. Also in higher concentrations the decongestant part is banned.
FWIW, I take Allegra and Allegra D, have raced on both and experience no problems with either.