A homoeopathic aggravation is a noticeable heightening of disease symptoms which were originally observed, and which occurs shortly after a homoeopathic dose, especially during treatment of chronic disease.
From the law of similars, similar symptoms of the disease from which the patient is suffering are induced by the homoeopathic medicine, and so it follows that there can be a slight worsening of the symptoms before a cure.
Sometimes it is said that an aggravation is a sign that a homoeopathic remedy is doing its job. If an aggravation occurs, it may only be a slight worsening of symptoms or a transient change, and may hardly be noticed by the patient. As Hahnemann said "A slight homoeopathic aggravation in the first hours is a good portent that the acute disease will probably be cured". (Organon, paragraph 158).
If a homoeopathic aggravation is marked, or if it persists, this may because the patient is proving the remedy, because a remedy that is poorly matched may actually induce new symptoms in the patient.
"An appropriately chosen homoeopathic remedy gently removes the disease without arousing any new symptoms. It is, nevertheless, usual in the first few hours if the dose is too large (too low a potency) for it to effect some small aggravation" (Organon, paragraph 157). "Aggravations do not occur if the accurately chosen homoeopathic medicine is given in low potencies which are gradually increased and modified by higher potencies" (Organon, paragraphs 161 and 247).
High potencies may also cause aggravations due to the high energy which may over stimulate an already weakened defense system.
To quote from Hahnemann: "If the symptom list of the nearest homoeopathic medicine contains the greatest number of symptoms to be covered and the disease being treated, this specific medicine is the most suitable in this case, with no significant side effects" (Organon, paragraph 154). "I repeat, no significant side effects, because when the most suitable homoeopathic medicine is used, only the medicinal symptoms of the remedy which correspond to the disease act" (Organon, paragraph 155).
"Nevertheless, there is hardly any homoeopathic medicine, even well chosen, which during its action does not bring about in very sensitive patients some very slight new symptom, because it is almost impossible for the symptoms of the medicine to coincide" (Organon, paragraph 156).
From this it can be seen that homoeopathic aggravations can be a necessary part of homoeopathic treatment, but if the choosing of the remedy and the potency are careful, then they will usually not prove to be a problem.