Some treatments may cause undesired or dangerous reactions in patients with allergies, high blood pressure, or are pregnant.
In addition to this many medications should not be combined.There are two types of contraindications:
‘Relative contraindication’ means that care should be taken when two drugs or procedures are used in conjunction.
‘Absolute contraindication’ means that a life-threatening situation may be induced. This procedure or medication is generally avoided in medicine.

See also:
Drug-drug interaction
Compassionate use
Adverse Reaction