Loratadine is a second generation antihistamine. This drug is believed to act by antagonizing the H1 receptor. As with all antihistamines, you may experience the side effects of dry mouth/throat, headache, fatigue, urinary retention, blurred vision and drowsiness. Loratadine is probably less sedating than some of the first generation antihistamines. This is not meant to be an all-inclusive list of side effects. Several of these side effects can be attributed to blockade of cholinergic muscarinic receptors.
You should know that loratadine is metabolized by cytochrome P450 liver enzymes. This is significant in that several other drugs (long list of various antibiotics, antifungals, etc.) affect the same liver enzymes. Simultaneous use of these meds can result in higher concentrations of the loratadine hanging around for longer periods of time in your body. For this reason, it can be helpful to ask your pharmacist or doctor whether loratadine interacts with any other meds you are taking.
That being said, I know several divers who regularly take loratadine (Claritin) or cetirizine (Zyrtec) for their allergies. None of them have reported any issues with those meds while diving.
As with all new medications, try taking loratadine on a non-diving day first to see how the med affects you. It's interesting to note that many of the symptoms divers attribute to diving or being on a boat (dry mouth/throat, headache, fatigue) may be very similar to the side effects of antihistamines.
