Discussion about allergic reactions to medicines.

Overall frequency of allergic reactions to medicines is around 10%.It is described a predisposition that is probably genetically determined. Allergic sensitization usually develops after 5 to 14 days after the first administration, sometimes longer, if the treatment is continuously.
The risk is higher for local administration and less for oral administration. The allergy is specific  to a substance, but may also include structurally related compounds. Reactions are common to all penicillins for example.
It is necessary for the development of allergy that the medicine,which is generally with low molecular weight, to form conjugates with covalent binding of a polypeptide or protein in the body. Sometimes antigenic determinant is a metabolite of the drug (eg Peniciloil derivatives for  penicillin).
In the U.S. approximately 50% of approved drugs had adverse effects undetected in the testing stages prior to approval, in the same country adverse drug reactions affecting 2.2 million patients and is the fifth - the sixth cause of death (0.1% of patients admitted to medical wards).

Rarely is it possible to remove a specific morbid conditions through  pharmacon without being influenced concurrently and other functions of the body (side effects). Side effects may include: desired / unwanted, harmful /non harmful,  dose dependent, predictable / unpredictable. 
Classification of adverse effects / side:

A  pharmacodynamic causes undesirable effects

a. Effects on overdose (increasing of main effect  , the emergence of new effects, emphasizing the different effects of the master, but inseparable)
b. inevitable effects at therapeutic doses
1. because the main action:
- By the same mechanism as the main action (broncho obstruction after β-blockers);
- Following the main action (reflex tachycardia after hypotensive).
2. independent of the main proceedings (thrombosis after oral contraceptives).
B. undesirable effects of   pharmacokinetic causes(absorption, distribution, release / bioavailability, biotransformation, enzyme induction, elimination, aggregation)
C. Undesirable effects by drug interactions (pharmacokinetic / pharmacodynamic)

D. Undesirable effects due to age (young children, elderly)
E. Effects unwanted by genetic influences 
 
F. Effects unwanted immune mechanisms ( 'allergies')
 
G.Carcinogen  action 
H. Embryotoxic and teratogenic action