Loratadine works by preventing the actions of histamine - a substance produced by the body when it reacts to a foreign substance, such as pollen or pet fur (these are known as allergens).
Histamine causes a chain reaction that results in allergic symptoms such as itchy eyes, sneezing and a runny nose.
Drug companies are naturally pretty reluctant to go around testing medicines on pregnant or breast-feeding women, so there aren't any studies to guide them.
They tend to advise against to be on the safe side, although there is no evidence they cause a problem.
They have a number of uses, but are most often used to treat allergies.
If you are one of the many people who get hay fever each year, you will know all about antihistamines. Some of these medicines are available to buy over the counter from your pharmacist. These medicines come in a variety of forms, as mentioned above.
For hay fever, sometimes if the antihistamine tablet isn't enough by itself, you may need eye drops and/or a nasal spray as well to completely sort out all your symptoms.
Other than these, loratadine is not known to significantly affect other medicines.
Usually it is possible to find an antihistamine which keeps your symptoms controlled.
Sometimes you may need to experiment and try more than one to find the one that works best for you.
This is one of the most common uses of antihistamines. Your doctor or pharmacist will advise you on how to take your medication, including what dose and how often.
Read the leaflet that comes with your particular brand for further information.