A 5 mg dose of Claritin (loratadine) is pretty low. If after an hour or two you don't notice any reduction in your symptoms, it won't hurt you to take the Zyrtec immediately.
Be aware that just Zyrtec alone can make you drowsy and impair your ability to drive or operate machinery; Zyrtec and Claritin together will amplify this effect, so please use caution to avoid a situation where you may injure yourself or others.
Also, no oral antihistamine will rapidly eliminate your symptoms completely. How long the hives reaction lasts depends on what is causing them, whether you are still being exposed to whatever is causing them, and whether you scratch and scrub the area (which irritates the skin even more).
While antihistamines like Claritin and Zyrtec are good options, we have listed some alternatives below that may work faster for acute symptoms.
Treatment For Hives
You may have more success with a topical cream or ointment containing
hydrocortisone or diphenhydramine, or the anti-itch lotion Sarna. This is
because drugs taken orally have more difficulty making it to the outer layers
of your skin where the reaction is occurring. Topicals are applied directly to
the reaction site, allowing them to act immediately and for a longer duration.
To use a topical preparation, gently wash the area with lukewarm water and soap, preferably unscented soap without extra stuff in it. Use your hands to wash the area...don’t use a cloth or poof, and don’t scrub. Rinse clean and pat (not rub) dry with a clean towel. Your skin doesn’t need to be perfectly dry, and by the time you get the medication out of the package, you should be ready to apply. Apply the recommended amount using your hands, gently rub it in until mostly absorbed, then wash your hands.
If the hives reaction covers a large portion of your body (larger than your hand with your fingers spread as far apart as possible), check with a doctor before using a topical preparation.
If you've had hives before, but don't know what's causing them, it would be a good move to contact an allergist or other physician who can perform a scratch/stamp test. This test exposes you via small scratches in your skin to tiny amounts of a wide variety of substances that humans are often allergic to.
From the level of reaction on your skin, the doctor can tell what you are allergic to, and how severe your reaction is. This will help you to avoid future episodes by reducing your exposure to the offending substance.