My Dr's advice today was taking the odd one here or there, or for a couple of days or so at a time is fine. Not such a good idea if it's for an extended period though, such as taking one every day for weeks on end. Or more.

Though unfortunately that doesn't really help with severe or extended hayfever/allergy problems. Suggested eye-drops, nasal sprays etc can be used as an alternative and keep the tablets for when it gets super bad.