The concentration of a drug injected into the bloodstream decreases with time. The intervals of time $T$ when the drug should be administered are given by

$$T=\frac{1}{k} \ln \frac{C_{2}}{C_{1}}$$

where $k$ is a constant determined by the drug in use, $C_{2}$ is the concentration at which the drug is harmful, and $C_{1}$ is the concentration below which the drug is ineffective. (Source: Horelick, Brindell and Sinan Koont, "Applications of Calculus to Medicine: Prescribing Safe and Effective Dosage," UMAP Module 202.) Thus, if $T=4,$ the drug should be administered every 4 hr. For a certain drug, $k=\frac{1}{3}, C_{2}=5,$ and $C_{1}=2 .$ How often should the drug be administered? (Hint: Round down.)