non-ionized forms of drugs are usually easier to absorb because they will not be repelled by the lipidic bilayer of the cell, most of them can be absorbed by passive diffusion, unless they are too large or polarized (like glucose or vancomycin), in which case they may or may not have specific and non-specific transporters distributed on the entire intestine internal surface, that carries drugs inside the body. Obviously increasing the absorption of a drug will increase its bioavailability, so, changing the drug's state between ionized or not can be useful for certain drugs. Certain drugs require an acid stomach pH for absorption. Others require the basic pH of the intestines. Any modification in the pH could change this absorption. In the case of antacids, an increase in pH can inhibit the absorption of other drugs such as zalcitabine (absorption can be decreased by 25%), tipranavir (25%) and amprenavir (up to 35%). However, this occurs less often than an increase in pH causes an increase in absorption. Such occurs when cimetidine is taken with didanosine. In this case, a gap of two to four hours between taking the two drugs is usually sufficient to avoid the interaction. Drug solubility: The absorption of some drugs can be drastically reduced if they are administered together with food with high-fat content. This is the case for oral anticoagulants and avocado. Formation of non-absorbable complexes: Chelation: The presence of di- or trivalent cations can cause the chelation of certain drugs, making them harder to absorb. This interaction frequently occurs between drugs such as tetracycline or the fluoroquinolones and dairy products (due to the presence of Ca++). Binding with proteins. Some drugs such as sucralfate binds to proteins, especially if they have a high bioavailability. For this reason its administration is contraindicated in enteral feeding. Finally, another possibility is that the drug is retained in the intestinal lumen forming large complexes that impede its absorption. This can occur with cholestyramine if it is associated with sulfamethoxazol, thyroxine, warfarin or digoxin. Acting on the P-glycoprotein of the enterocytes: This appears to be one of the mechanisms promoted by the consumption of grapefruit juice in increasing the bioavailability of various drugs, regardless of its demonstrated inhibitory activity on first pass metabolism. Based on transport and distribution The main interaction mechanism is competition for plasma protein transport. In these cases the drug that arrives first binds with the