Lethal dosis needs to be determined, which is called LD50 (meaning at this dosage 50% of test animals died). Then lower dosages are tried to show effects. The usable dosage is called "therapeutical range" which describes the dosage range in which the desired effect takes place without being toxic. The broader this range, the safer the med. Many meds have a tight range which means the dosage needs to be monitored and adjusted closely (e.g. certain seizure meds).