Dose could be defined as the least quantity of any substance required to affect a change in nature. The science or doctrine of dosage is called is called posology.

In Greek posos =how much ) .
In medical parlance dose could be defined as the quantity of a particular medicine administered to an individual at a time .

Doses can be
1) Physiological dose
A dose of drug empirically selected of sufficient quantity and strength to produce definite pre-determined effect or a group of symptoms.
For eg physiological dose of belladonna is one which is sufficient to produce dialatation of pupil, dryness of mucus membrane and flushing and turgescence of skin. This is the quantity of medicine which could be administered with safety.

2) Pathological dose
Quantity of medicine capable of producing pathological changes in the tissues of the organisum. i.e. biochemical changes, bio physical changes .

3) Toxic dose
Dose which produces toxic effects upon the organisum.

4) Lethal dose
Dose which produces deletrious effects upon the organisum even death.

5) Therapeutic dose
Least quantity of medicine required to effect a cure or a palliation.
In homoeopathy therapeutic dose is the infinitesimal dose or the minimum dose or the sub- physiological dose .

5) Minimum dose
In $ 280 minimum dose is defined as the quantity of medicine required to produce a scarcely perceptible homoeopathic aggravation.

According to Stuart Close minimum dose is a dose which is not capable of producing symptoms when used therapeutically Minimum dose is otherwise known as sub physiological dose or homoeopathic dose.

Evolution of the concept of minimum dose by Hahnemann
Time period through which the concept of minimum dose evolved in Hahnemann ’s mind could be separated into three phases .

Period - (1779 to 1799 )
Hahnemann was disappointed and disgusted by the practice of applying large and heroic doses of medicine which he found was only destroying the health .He raised his voice against the existing dosage of medicine in his article on the treatment of veneral disease
Mercurius was given at that time 1 to 5 grams according to the severity of the disease .Hahnemann advised against this and said that only ¼ to 1 grain is sufficient for the treatment of the same.
In the cure and prevention of scarlet fever Hahnemann speaks about the dose of opium to be given . He advises 1 part of opium to be taken in 20 parts of weak alcohol and keep it for one day , then one drop of it is taken and dissolved in 500 drops of alcohol and one drop of it could be given for children and 2 drops for persons above 10 years of age . (dose is 1/ 20 x 500 ) . Here we can see that a reduction of dose was insisted by Hahnemann.

Period ( 1799 to 1827 )
It was a time period in which Hahnemann was treating some cases homoeopathically and some cases according to old system of medicine . In a\c and c\c cases Hahnemann shifted to the smallest doses which were of the second and third dilution .In medicine of experience he says that the dose must be small in accordance to the disease , so looking at the nature of the disease dose has to be determined .

Period ( 1828 onwards )
This third period dawns with the arrival of Hahnemann’s chronic disease . Hahnemann became assured of the efficacy of minimum doses in treatment of disease . He fixes the dose as a globule not bigger than that of a poppy seed . In the 5th edition of organon he says that a dose could be as minimum as 1/ 200 of a grain . In olfaction 1/400 of a grain should be used .

Observations of Hahnemann that lead to the derivation of the concept of minimum dose
Dr Dudgeon in his Lectures and practice of homoeopathy says that in deriving the concept of minimum dose -

a) Hahnemann observed that medicines exhibited greater strength when given in dilutions than in dry state .
b) He observed the greater power of medicine when given in divided doses than given at once .
c) He observed the greater susceptibility of disease organisum for the medicines having a specific homoeopathic relation to the affected parts .
d) He observed an increasing power of medicine by a thorough admixture of vehicle by means of succusion .
e) Desire to evade persecussion of apothecaries who tried legal proceedings against Hahnemann for invading upon their privileges for dispensing medicine .
f) To avoid aggravation of disease when given in large doses .

Why dose should be minimum
1) When the disease attacks the body , it overcomes the body resistance . Now the body becomes vulnerable to the action of a similarly acting disease producing agent . So this disease producing agent viz the drug need only be applied in a minimum dose just sufficient to produce a cure .

2) Disease has already rendered the parts abnormally sensitive , so only a slight stimuli need be applied in order to reduce aggravation
3) According to Arnold - Schutz minimum dose stimulates , medium inhibits and maximum destroys .

GUIDELINES BY DIFFERENT AUTHORS

Hahnemann
In his organon of medicine he says
$279 The dose of homoeopathically selected remedy can never be prepared so small that it shall not be stronger than natural disease .
$280 fn
Let them learn from the mathematicians how true it is that a substance divided into so many parts still contains its smallest part and the smallest part dose not cease to have some substance
$ 281
Every patient ever so robust will be effected by small conceivable dose .
An exception to infintesimal dose is given
In $ 282 fn there is an exception in the treatment of three great miasm while they still efflorescence on the skin ie recently erupted itch ,the untouched chancre and the fig warts . These not only tolerate , but indeed require from the very beginning large doses of their specific remedies of ever higher and higher dynamisation daily

Stuart close -
According to Stuart Close there are 3 necessary requirements for the action of infinitesimal dose .
1) The development of special virtues of medicine by the process of potentisation .
2) The increased susceptibility to medicinal impression produced by the disease .
3) The selection of symptomatically similar remedy .

H.A. Robert
According to H A Roberts the law of dosage could be summarized from 3 fundamental laws .
1) Law of least action and quantity
The quantity of action necessary to effect any change in nature is least possible . The decisive amount is always the minimum ie an infinitesimal
2) Law of quantity and dose
The quantity of drug required is inverse ratio to its similarity .
3) Law of quality
The quality of action of homoeopathic medicines is determined by its quantity in its inverse ratio .

H A Roberts in The principles and art of cure of homoeopathy says that infinitesimial dose is one which is so small as not to produce too much aggravation of the symptoms already present and never large enough to produce new symptoms .

Carrol Dunham
In his Science of therapeutics he says that the most vigorous opposition from old school as well as chief ground of division among the homoeopaths was upon the question of infinitesimal dose .Dunham says that the question of minimum dose is manifestly an open one .He states in favour of infinitesimal dose and concludes that there are many evidences in chronic disease that many medicines have acted in very large doses .

Richard Hughs
Richard Hughs based his infinitesimal in those doses which which could be considered as very large .


Ref books :
1) The genius of homoeopathy - Stuart Close
2)The principles and art of cure by homoeopathy – H A Roberts
3)The science of therapeutics - Caroll Dunham
4) Lectures on the theory and practice of homoeopathy - R E Dudgeon
5) Organon of Medicine