Over the counter allergy medications are almost all long acting non sedating antihistamines these days. These medications are very safe and they have very few side effects. It is not recommended that you take more than one 24 hour pill per day. Doing so may cause some drowsiness or dry mouth, among other minor side effects. However, more importantly, there is no evidence that doubling the dose is any more effective than 1 pill per day at relieving allergy symptoms.
In other words, if you are taking your allergy medicine as directed and not having relief of your symptoms, then it is time to set up a visit with your primary care doctor
for help. In these situations, it is often necessary to add
a second medication in addition to the over the counter allergy medicine.
Depending on which physical findings your doctor identifies in the visit, they will be able to recommend which medication that should be. For example, if you have mostly symptoms of nasal congestion, then a prescription steroid nasal spray might be the best option. On the other hand, if your symptoms are mostly eye redness and itching, then prescription eye drops might be a better choice.