Hi Elaine
Typically, the artificial tears that you find in pharmacies such as Refresh/Refresh Plus or Tears Naturale contain very plain/neutral substances such as carboxymethylcellulose or polyvinyl alcohol, which serve functions such as to thicken the eyedrop and keep it around the eye a bit longer. They may also contain minerals/electrolytes to keep the osmolarity (concentration of salts) similar to tears, so that the artificial tears do not sting when instilled.
I would not expect any of these substances to have any side effects, whether in the short or long term. So yes, they can be used indefinitely.
Some other eyedrops such as Prefrin contain 'eye-whiteners' and it would be best to minimise using these to avoid 'rebound redness' on stopping their usage.
