Long term absence will normally fall into the context of long term illness. A return to work plan can be agreed between long-term sick employees and their employer which avoids the need to keep obtaining fit notes.
Persistent or intermittent absence (ie where an employee keeps taking short amounts of time off for sickness or another reason) should be monitored and assessed over time.
Employers should consider if long-term sickness or persistent short-term sickness constitutes a disability; if so, they may be required to make reasonable adjustments.
Ultimately, employers can dismiss a long-term sick employee. However they must first consider if the employee is able to return to work (eg in a more flexible capacity) and they should also consult with the employee regarding the outlook of their condition and their prospect of returning to work. For employees who are persistently absent (due to illness or for other reasons), care should be taken when considering dismissal and the relevant internal policies should be followed.