which trigger compensation mechanisms thereby increasing blood glucose levels. The release of catecholamines also triggers a series of symptoms, which allows the organism to recognize what is happening and which act as a stimulant for preventative action (eating sugars). Should a patient be taking a drug such as insulin, which reduces glycemia, and also be taking another drug such as certain beta-blockers for heart disease, then the beta-blockers will act to block the adrenaline receptors. This will block the reaction triggered by the catecholamines should a hypoglycaemic episode occur. Therefore, the body will not adopt corrective mechanisms and there will be an increased risk of a serious reaction resulting from the ingestion of both drugs at the same time. Antagonistic physiological systems: where drugs taken together cause adverse reactions because the effect of one substance is indirectly increased in the presence of another. This can occur when a certain drug, which increases the presence of a physiological substance, is introduced into a system with another drug that is amplified by the same substance. An actual example of this interaction is found in the concomitant use of digoxin and furosemide. The former acts on cardiac fibers and its effects are increased if there are low levels of potassium (K) in blood plasma. Furosemide is a diuretic that lowers arterial tension but favors the loss of K+. This could lead to hypokalemia (low levels of potassium in the blood), which could increase the toxicity of digoxin. Based on pharmacokinetics Modifications in the effect of a drug are caused by differences in the absorption, transport, distribution, metabolism, or excretion of one or both of the drugs compared with the expected behaviour of each drug when taken individually. These changes are modifications in the concentration of the drugs. In this respect, two drugs can be homergic if they have the same effect on the organism and heterergic if their effects are different. Absorption Changes in motility Some drugs, such as prokinetic agents, increase the speed with which a substance passes through the intestines. If a drug is present in the digestive tract's absorption zone for a short amount of time, its blood concentration will decrease. The opposite will occur with drugs that decrease intestinal motility. pH: Drugs can be present in either ionized or non-ionized form, depending on their pKa (pH at which the drug reaches equilibrium between its ionized and non-ionized form). The