• Atonic (non-functioning) colon which may also result from long-term
  use of stimulant laxatives
• Diverticular disease (chronic); diverticulitis (acute attacks)
• Faecal impaction
• Intestinal obstruction (or even perforation)
• Faecal incontinence : may have overflow diarrhoea
• Urinary incontinence
• Urinary tract infection especially with E. Coli, a normal gut bacterium
• Rectal bleeding
• Cramping lower abdominal and back pain
• Generalised weakness
  

Straining to defaecate causes

• An increase in intrathoracic (chest cavity) pressure which can lead to a
  reduction in coronary (heart), cerebral (brain) and peripheral circulation.
• development of hernias,
• worsening of symptoms of gastro-oesophageal reflux (GORD),
• transient ischaemic attacks (mini-strokes) and syncope (fainting) in 
  patients  with neurological disease
• Haemorrhoids may develop due to straining.
  

There has been an association between chronic constipation and
cancer of the colon suggested in the literature; however, the evidence for
this is not conclusive.

Recommending a high fibre, low-red meat diet and regular exercise in
itself has been recognised in helping to reduce the risk of colon cancer as
well as being beneficial in combating constipation.