All drugs bring about complex interactions with molecules of living systems. These actions, either physical or chemical, mainly act at four levels:
- Molecular– Receptor, ion channel, enzyme, carrier molecules
- Cellular– Transduction e.g., G protein, ion channel, enzyme
- Tissue– Contraction, secretion, metabolic activity, proliferation
- System– CNS, CVS , etc.
The target molecules where the drugs act are linked with various biochemical reactions in the cells, resulting ultimately in response. These may be enzyme linked, Ca linked or g-protein linked.
When the drug effect occurs in immediate vicinity of application, this is known as local effect.
When the drug effect occurs away from the site of administration, this is known as systemic effect.
Primary effect is the effect for which the drug is administered or the treatment of the disease for which the drug is given.
All other effects occurring in addition are known as secondary effects. When the secondary effects are undesirable by the patient, these are called adverse effects or side effects.
Most drugs act by altering the various body control systems, which may be receptors, enzymes or ion channels. These various mechanisms include: