Pharmacology is the study of how drugs act, from the submicroscopic molecular level to statistical investigations of the effects of drugs on populations of people - pharmacoepidemiology.
Toxicology is largely a subset of Pharmacology and is glibly defined as Overdose Pharmacology.
A Pharmacologist is a scientist who discovers how drugs cause their actions. Pharmacologists quantitate mechanism by analyzing the dose-response for therapeutic and toxic effects. A Toxicologist is attentive to adverse reactions to drugs.
A Pharmacologist is different from a Pharmacist - the latter is a healthcare professional who dispenses them to patients based on the research performed by pharmacologists. It is the job of the pharmacologist to discover drugs and determine their benefits, risks, safety, side effects, and adverse reactions.