During renal excretion, as the glomerular filtrate reaches the DCT, highly permeable lipophilic and nonionized drugs are passively reabsorbed from the tubular fluid into the peritubular capillaries. This tubular reabsorption limits their renal excretion. Most drugs, however, are weak acids or weak bases, and the extent of their ionization is pH dependent. By manipulating the pH of the urine, reabsorption of such drugs can be prevented. When the urine is made alkaline, weakly acidic drugs are largely ionized. As the ionized forms are less permeable through the tubular cells, they are retained in the lumen and eventually excreted in the urine. This process of 'ion trapping' prevents reabsorption and increases renal clearance of undesirable drugs. For instance, an overdose of phenobarbital —a weak acid—is treated with bicarbonate which makes the urine alkaline. This ionizes the drug and prevents its back diffusion. Similarly, an overdose of weakly basic amphetamines, is treated by acidifying the urine such that the ionized drug does not get reabsorbed.