C. Median toxicity.
Explanation – The frequency of drugs are given based on its toxicity or the side effects in the body. All drugs have certain half-life in which there action is maximum and after that the drug is removed through the urine primarily.
Different people react differently to a drug and hence the doctors prescribe the anti-tablets for the side effects occurring. The drug for cold is prescribed after four hours because its effect gets reduced and has to be removed from the body.