A small amount can induce hyper-excitability for a time. Larger doses lead to sedation, sleep or even death. Strychinine which causes convulsions in large doses can facilitate animal learning in smaller doses. You must know the dose to predict drug effects. The dose response curve describes the relationship between the amount of the drug given and the degree of response produced by the drug.
Body weight is the primary factor that affects drug dose.
An individual dose response curve plots a single drug effect on a single individual at different dosages. The threshold response is the point at which the measurable response is first observed. A drug will produce different dose response curves for each of its effects. Curves are generally unique to each individual. A group drug response curve plots the percentage of several individuals showing the desired effect at various dosage levels.