Pharmaceutical solutions

Pharmaceutical solutions

Introduction

Solutions are homogeneous mixtures of one or more solutes molecularly dispersed in a suitable solvent or a mixture of mutually miscible solvents. A solution composed of only two substances is a binary solution. The com-ponents making up a binary solution are termed the solute and the solvent depending on their relative proportions (component in lower proportion is termed solute).

Pharmaceutical solutions are used for many routes of administration, including oral, rectal, vaginal, ophthalmic, parenteral, and otic. The most common solution dosage form is the oral liquid, which includes aque-ous solutions, syrups, and elixirs. The physicochemical (e.g., solubility) and stability characteristics of the active drug determine whether an oral solution dosage form can be prepared. The required solubility of a drug and its solubility in water and biocompatible water-miscible solvents help decide the dosage form composition. For example, if the drug is water sol-uble, a simple aqueous solution can be prepared. However, if it is soluble in a water–alcohol–glycerin cosolvent system, an elixir is appropriate.

Drugs are commonly given in solution in cough/cold remedies and in medi-cations for the young (pediatric) and elderly (geriatric).

Saturated solutions are solutions which, at a given temperature and pressure, contain the maximum amount of solute that can be dissolved in the solvent. Buffer solutions contain a combination of weak acid and its salt with a strong base or a weak base and its salt with a strong acid. These solutions resist changes in pH upon the addition of small quantities of acid or base. Solubility and stability of most ionic drugs change with pH. Therefore, most pharmaceutical solutions are pH controlled using an appropriate buffer. Isotonic solutions have similar tonicity as biological fluids. These solutions cause no swelling or contraction of the tissues with which they come in contact and produce no discomfort when instilled in the eye, nasal tract, blood, or other body tissues. Parenteral solutions or solutions for direct administration to mucosal tissues should be isotonic or hypotonic to avoid local tissue stress and pain upon administration. Tonicity is usually adjusted using dextrose or sodium chloride. Isotonic sodium chloride is a 0.9% w/v concentration of NaCl in water and is also called normal saline. A 5% w/w dextrose, also known as glucose, solution in water is also isotonic. These are commonly used infusion fluids for intra-venous administration.

Solutions intended for oral administration usually contain sweeteners, flavors, and colors to make the medication more attractive and palatable to the patient. They may contain stabilizers to maintain the physicochemical stability of the drug and preservatives to prevent the growth of microorgan-isms in the solution.

A drug dissolved in an aqueous solution is generally in the most bioavail-able form. As the drug is already in solution, no dissolution step is neces-sary before systemic absorption occurs.