Mechanisms
As research better explains the biochemistry of drug use, fewer ADRs are Type B and more are Type A. Common mechanisms are:

Abnormal pharmacokinetics due to
genetic factors
comorbid disease states
Synergistic effects between either
a drug and a disease
two drugs
Antagonism effects between either
a drug and a disease
two drugs

Abnormal pharmacokinetics
Comorbid disease states
Various diseases, especially those that cause renal or hepatic insufficiency, may alter drug metabolism. Resources are available that report changes in a drugs metabolism due to disease states.The Medication Appropriateness Tool for Comorbid Health Conditions in Dementia (MATCH-D) criteria warns that people with dementia are more likely to experience adverse effects, and that they are less likely to be able to reliably report symptoms. Genetic factors
Abnormal drug metabolism may be due to inherited factors of either Phase I oxidation or Phase II conjugation. Pharmacogenomics is the study of the inherited basis for abnormal drug reactions. Phase I reactions
Inheriting abnormal alleles of cytochrome P450 can alter drug metabolism. Tables are available to check for drug interactions due to P450 interactions.Inheriting abnormal butyrylcholinesterase (pseudocholinesterase) may affect metabolism of drugs such as succinylcholine

Phase II reactions
Inheriting abnormal N-acetyltransferase which conjugated some drugs to facilitate excretion may affect the metabolism of drugs such as isoniazid, hydralazine, and procainamide.Inheriting abnormal thiopurine S-methyltransferase may affect the metabolism of the thiopurine drugs mercaptopurine and azathioprine. Interactions with other drugs
The risk of drug interactions is increased with polypharmacy. Protein binding
These interactions are usually transient and mild until a new steady state is achieved. These are mainly for drugs without much first-pass liver metabolism.
An adverse drug reaction (ADR) is a harmful, unintended result caused by taking medication. ADRs may occur following a single dose or prolonged administration of a drug or result from the combination of two or more drugs. The meaning of this term differs from the term "side effect" because side effects can be beneficial as well as detrimental. The study of ADRs is the concern of the field known as pharmacovigilance. An adverse drug event (ADE) refers to any unexpected and inappropriate occurrence at the time a drug is used, whether or not associated with the administration of the drug. An ADR is a special type of ADE in which a causative relationship can be shown. ADRs are only one type of medication-related harm, as harm can also be caused by omitting to take indicated medications. Classification
ADRs may be classified by e.g. cause and severity. Cause
Type A: Augmented pharmacologic effects - dose dependent and predictableType A reactions, which constitute approximately 80% of adverse drug reactions, are usually a consequence of the drugs primary pharmacological effect (e.g. bleeding when using the anticoagulant warfarin) or a low therapeutic index of the drug (e.g. nausea from digoxin), and they are therefore predictable. They are dose-related and usually mild, although they may be serious or even fatal (e.g. intracranial bleeding from warfarin). Such reactions are usually due to inappropriate dosage, especially when drug elimination is impaired.