what is posology(doses of drugs) and its types?posology

POSOLOGY

It the branch of pharmacology which deals with the study of the dosage of drugs is called posology.
  Dose is the quantity of drug given to the patient at one time.The quantity should be adequate to produce certain therapeutic effects.It should not be too small or too large.If too large, undesirable effects will appear.However there are individual variations and the same drug in the same doses may show marked differences in response.Some persons may be highly sensitive and respond to a very small dose, while others are more resistant and fail to respond except in very large doses.This is known as “normal biological variation”.

posology (doses of drug)

  The British Pharmacopoeia gives two doses, one a minimum and the other a maximum dose. By “minimum dose” is meant the minimum amount which is expected to produce some therapeutic effects. By “maximum dose” is meant the largest amount which one can use without risk of producing toxic effects.These doses are meant for oral administration unless otherwise stated, and are for adults i.e. for persons between 18 and 60 years. When a drug is meant for administration by any other route, the dose to be used as well as the route of administration by any other route, the dose to be used as well as the route of administration is specifically mentioned.
The United States Pharmacopoeia gives only an “average dose”. It is for the prescriber to adjust the dose according to the requirements of the patient.

Daily dose:

   It is the total amount of the drug which can be given in the day either as a single dose, as with emetine hydrochloride, or the total amount is to be divided into three or four doses.The total daily dose of sodium amino salicylate is 10 to 15 g and it should be given in divided doses.

Total dose:

     It is the amount of the drug to be given to the patient during the full course of treatment. It should not be exceeded on account of toxicity. Thus the dose of colchicine is 1/2 to 1mg. Total dose is 8 mg.

Single dose:

     Means that only one dose should be given during the day. Single dose of Emetine is 30 to 60mg.
      For the treatment of certain diseases it is necessary to use a large dose initially to produce an effective concentration as quickly as possible and then to use small dose to maintain the effect. The former is known as “initial or loading dose” and the latter as “maintenance dose”. The initial dose of sulphadiazine is 3 g and subsequent dose is 1 g. Initial dose of digoxin is 1 to 1.5 mg and the maintenance dose is 0.25 mg once or twice daily.
    For certain drugs the British Pharmacopoeia does not prescribe any dose and expects the physician to select the dose according to the requirements of the patients; for example, no dose is given for insulin and some of the antibiotics.

Median Lethal Dose(LD50):

         It is the amount of drug which is fatal to 50 percent of experimental animals. It is the measure of acute toxicity of the drugs.

Median Effective Dose(ED50):

     It is the amount of the drug which produces the desired therapeutic effect in 50 percent of experimental animals. It is the measure of effectiveness of the drug.

Therapeutic Index(Therapeutic Ratio):

       It is the ratio of median lethal dose to the median effective dose. The higher is the ratio the safer is the drug.

Toxic Dose:

   It is the amount of drug which produces undesirable harmful effects of serious nature and liable to endanger the life of the patient.

Fatal Dose:

   It is the amount of drug which is liable to cause death due to toxic effect of the drug in a normal person.

Also Read
                   What is pharmacology?history of pharmacology
                     What are Alpha blockers?side effects of alpha blockers?