Posology is a vital part of medical and pharmaceutical practice and study, not just for humans, but also for animals. It is the science of determining and understanding drug dosage, as based on research into a huge number of factors. To a limited or extensive degree, posology may be studied by anyone who wants to become a doctor, nurse, pharmacist, veterinarian, or pharmacologist.

**Calculationsin Pharmacology **

Medicationsare great, and they have been valuable to the sum of veterinary and humanmedicine. They may cure some diseases completely, and they may greatly reducesymptoms of others. The issue of exactly what amount to give is one that mustbe carefully considered. Digitalis, for instance, can improve heart function,and it can kill people easily. Deciding how to give the right dose is thusextremely important, and those administering this drug need to know how toprescribe it appropriately. In order to write a prescription, Physician musthave understanding about the units used in weighing and measuring the medicinalagents.

Metricsystems of units have evolved since the adoption of the first well-definedsystem in France in 1795. During this evolution the use of these systems hasspread throughout the world. Multiples and submultiples of metric units arerelated by powers of ten and their names are formed with prefixes. Thisrelationship is compatible with the decimal system of numbers and itcontributes greatly to the convenience of metric units. In the early metricsystem there were two fundamental or base units, the metre for length and thegram for mass. The other units of length and mass, and all units of area,volume, and compound units such as density were derived from these twofundamental units.

In 1944 the council on pharmacy andchemistry of the American Medical Association adopted the metric systemexclusively. The advantage of the metric or decimal system, and its simplicity,brevity and its adoptively to every day need are now conceded universally. Inmany experimental procedures, including some in the pharmaceutical sciences,very small quantities (and occasionally very large) of the weight, length,volume, time or radioactivity are measured. To avoid the use of various numberswith many zero in such cases, the NIST recognized prefixes to be used toexpress fractions or multiple of the International System of Units, which wasestablished in 1960 by the General Conference on Weights and Measures therecognized prefixes, which in used are adjoined to an appropriate unit (as, forexample, in such quantities as nanogram, picomole, microcurie, microsecond, or,megavolt) are defined as under in table 1.

S.NO | FACTIONS | PREFIX | SYMBOL | MULTIPLE | PREFIX | SIMBLE |

1 | 10 ^{-1} | deci | d | 10 | deca | da |

2 | 10 ^{-2} | centi | c | 10 ^{2} | hecto | h |

3 | 10 ^{-3} | milli | m | 10 ^{3} | kilo | k |

4 | 10 ^{-6} | micro | µ | 10 ^{6} | miga | M |

5 | 10 ^{-9} | nano | n | 10 ^{9} | giga | G |

6 | 10 ^{-12} | pica | P | 10 ^{12} | tera | T |

7 | 10 ^{-15} | femto | F | 10 ^{15} | peta | P |

8 | 10 ^{-18} | atto | a | 10 ^{18} | exa | E |

Some of the metric weights are listedin the table 2 below. The prefixes, which indicate multiples, are of Greekderivation: deka, 10; hector, 100; kilo,1000. Fractions of the units areexpressed by Latin prefixes: deci, 1/10; centi, 1/100; milli, 1/1000.

S.NO | METRIC WEIGHT | EQUAL TO | EQUIVALENT WEIGHT |

1 | 1 microgram µg | = | 0.000001g |

2 | 1 milligram mg | = | 0.001g |

3 | 1 centigram cg | = | 0.01g |

4 | 1 decigram dg | = | 0.1g |

5 | 1 gram g | = | 1g |

6 | 1 decagram dag | = | 10g |

7 | 1 hectogram hg | = | 100g |

8 | 1 kilogram kg | = | 1000g |

S.NO | UNIT | INCHES | MM | µM | NM | A |

1 | 1 inch | 1 | 25.4 | 25400 | 2.54×107 | 2.54×108 |

2 | 1mm | 0.0394 | 1 | 1000 | 10 ^{6} | 10 ^{7} |

3 | 1µm | 3.94×10 ^{-5} | 10 ^{-3} | 1 | 1000 | 10000 |

4 | 1nm | 3.94×10 ^{-8} | 10 ^{-6} | 10 ^{-3} | 1 | 10 |

4 | 1A(Angstrom) | 3.94×10 ^{-9} | 10 ^{-7} | 10 ^{-4} | 0.1 | 1 |

S.NO | METRIC LEQUID MEASURES | EQUIVALENT |

1 | 1 microliter(µL) | 0.000001L |

2 | 1 mililiter(mL) | 0.001L |

3 | 1 centiliter(cL) | 0.01L |

4 | 1 deciliter(dL) | 0.1L |

5 | 1 liter(L) | 1L |

6 | 1 dekaliter(dL) | 10L |

7 | 1 hectoliter(hL) | 100L |

8 | 1 kiloliter(kL) | 1000L |

Thissystem grew out of the creative way that people measured for themselves. Familiar objects and parts of the body were used as measuring devices. For example, people measured shorter distances on the ground with their feet.These measures had their origins in a variety of cultures –Babylonian,Egyptian, Roman, Anglo-Saxon, and Norman French. The ancient "digit,""palm," "span" and "cubic" units of length slowlylost preference to the length units "inch," "foot," and"yard."

Romancontributions include the use of 12 as a base number (the foot is divided into12 inches) and the words from which we derive many of our present measurementunit names. For example, the 12 divisions of the Roman "pes," or footwere called unciae. Our words "inch" and "ounce" are bothderived from that Latin word.

The "yard" as a measure of length can be traced back toearly Saxon kings. They wore a sash or girdle around the waist that could beremoved and used as a convenient measuring device. The word "yard"comes from the Saxon word "gird" meaning the circumference of aperson’s waist.

In English system of measurement, **avoirdupois** and **apothecary** system of weight measurement are used in handlingmedicine. It must be emphasized that pharmacist may buy their drugs byavoirdupois weight.

The1878 Act said that contracts worded in terms of metric units would be deemed bythe courts to be made according to the Imperial units defined in the Act, and atable of metric equivalents was supplied so that the Imperial equivalents couldbe legally calculated. Thus defining, in UK law, metric units in terms ofImperial ones. The equivalence for the pound is given as 1 lb =453.59265 g or 0.45359 kg, which would make the kilogram weighapproximately 2.2046213 lb. In 1883, it was determined jointly by theStandards Department of the Board of Trade and the Bureau International that0.4535924277 kg was a better approximation, and this figure, rounded to0.45359243 kg was given legal status by an Order in Council in May 1898.However in 1963 a new Weights and Measures Act reversed this relationship andthe pound was defined for the first time as a mass equal to 0.45359237 kgto match the definition of the international pound agreed in 1959. To convertfrom the avoirdupois system to the metric system, one must know the conversionfactors, which are 1 gr = 65 mg, 1 g = 15.4 gr, 1 oz = 28.4 g and 1 kg = 2.2 lb.

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