General Weakness/Impairment:
* Weakening
* Enervating
* Incapacitating
* Crippling
* Disabling
* Exhausting
* Prostrating
* Draining
Severe or Chronic:
* Withering
* Devitalizing
* Shattering
* Devastating
* Harrowing
Mental/Emotional:
* Overwhelming
* Crushing
* Despairing
* Paralyzing
* Depressing
Specific to Illness/Injury:
* Lamentable (for a physical condition)
* Debilitating (for an illness)
* Enfeeblement (for a weakening condition)
* Ailment (for a minor ailment)
* Affliction (for a serious ailment)
The best synonym for "debilitating" will depend on the specific context and the nature of the weakness or impairment being described.