Rational and Irrational Combinations
Combining drugs is justified if it enhances the therapeutic effect or prevents adverse reactions. For example, adding vitamin B₆ to isoniazid prevents neurotoxicity, and combining antibiotics with levorin reduces the risk of candidiasis. Using potassium chloride along with diuretics (saluretics) helps avoid hypokalemia.
However, irrational combinations lead to incompatibility—where the therapeutic action is weakened, completely disappears, or prominent adverse reactions develop. Such interactions are divided into two major groups: pharmaceutical and pharmacological.
Pharmaceutical Interactions
Occur outside the body (ex vivo)—during manufacturing, storage, or immediately before administration (e.g., mixing in an IV bag or syringe).
These are based on physicochemical reactions:
- Formation of inactive or toxic compounds.
- Decreased solubility, precipitation, colloid coagulation.
- Stratification of emulsions, dampening of powders.
- Adsorption of one substance onto another.
This is usually visually apparent: the color, smell, or consistency of the mixture changes. Pharmacists monitor such prescriptions and perform pharmaceutical corrections to prevent the use of incompatible mixtures.
Pharmacokinetic Interactions
Develop inside the body (in vivo). One drug alters the concentration of the active form of another by interfering with its ADME processes (absorption, distribution, metabolism, excretion).
- At the absorption stage (in the GI tract): Dependent on pH changes (antacids alkalize the environment and hinder the absorption of weak acids, such as acetylsalicylic acid), formation of large complexes (activated charcoal binds other molecules), or altered peristalsis (cholinomimetics accelerate motility, reducing absorption time).
- At the distribution stage: Competition for plasma protein binding occurs. For example, sulfonamides displace tolbutamide from protein complexes, increasing its free fraction, which can lead to dangerous hypoglycemia.
- At the metabolism stage: Drugs can accelerate the activity of microsomal liver enzymes (inducers, such as phenobarbital), which weakens the action of other drugs. Conversely, inhibitors (verapamil) slow down drug degradation, causing their accumulation and a risk of toxicity.
- At the excretion stage: The kidneys eliminate ionized forms of substances faster. Acidic urine accelerates the excretion of weakly basic compounds (morphine), while alkaline urine accelerates weak acids (barbiturates). This principle is utilized in forced diuresis.
Pharmacodynamic Interactions
Occur at the level of the mechanism of action rather than drug concentration. They include two opposite phenomena:
- Synergism: Unidirectional action of drugs leading to an enhanced total effect.
- Antagonism: Reduction of effect. Can be receptor-mediated (competition of agonists and blockers, e.g., at adrenoceptors), at the level of mediators (influencing norepinephrine release), or functional (simultaneous intake of sleeping pills and psychostimulants).