I would like to believe that the major difference between these terms is based on their backgrounds and time however nowadays there are used interchangibly, without paying much attention to actual definitions.
The term chemotherapy was coined by Ehrlich at the beginning of the
20th century to describe the use of synthetic chemicals to destroy
infective agents. In recent years, the definition of the term has
been broadened to include antibiotics-substances produced by some
microorganisms (or by pharmaceutical chemists) that kill or inhibit
the growth of other microorganisms. It can also be broadened to
include agents that kill or inhibit the growth of cancer cells.
Chemotherapy is the term originally used to describe the use of drugs that are 'selectively toxic' to invading microorganisms while having minimal effects on the host.
The term also embraces the use of drugs that target tumours and, in fact, has now come to be associated specifically with that branch of pharmacology (Oncology). Although, in the public mind at least, chemotherapy is usually associated with cytotoxic anticancer drugs that cause unwanted effects such as loss of hair, nausea and vomiting.
Pharmacotherapy is a more recent term which generally means the treatment of diseases through the administation of drugs. However there is more to it, the term also describes a more patient-focussed approach in which pharmacists treat patients through drug management with one or more specific outcomes.
The specific goals of pharmacotherapy (primary therapeutic outcomes include):
• Cure of disease (e.g., bacterial infection).
• Reduction or elimination of symptoms (e.g., pain from cancer).
• Arresting or slowing of the progression of disease (e.g., rheumatoid
arthritis, HIV infection).
• Preventing a disease or symptom (e.g., coronary heart disease).
Other important outcomes of pharmacotherapy include:
• Not complicating or aggravating other existing disease states.
• Avoiding or minimizing adverse effects of treatment.
• Providing cost-effective therapy.
• Maintaining the patient’s quality of life.
A systematic approach is consistently applied to each case. The steps involved in this approach include:
Identifying real or potential drug therapy problems
Determining the desired therapeutic outcome(s)
Evaluating therapeutic alternatives
Hope this helps