Please give a overview about how the dosage of a particular drug (e.g. ciprofloxacin) is determined in pharmacology?
In general, the calculation of the first dose in man is based on No Observed Adverse Effect Level (NOAEL) determined in non-clinical safety studies performed in the most sensitive and relevant animal species, adjusted with allometric factors or on the basis of pharmacokinetics. The relevant dose is then reduced/adjusted by appropriate safety factors according to the particular aspects of the molecule and the design of the clinical trials.
In other words, the dosage is incremented (following special algorithms) until toxic effects are observed and then is reduced until an optimal effect/risk ratio is achieved. This procedure is most important during the "first-in-a-man" stage of drug development, but is also used in Phase 0 (lab work/animals).