All you need to know about Pharmaceutical Solutions

Pharmaceutical solutions are defined as liquid preparations containing therapeutic agents (active pharmaceutical ingredients) and the excipients dissolved in a solvent system. Pharmaceutical solutions are widely used as oral dosage forms (for oral administration). Solutions are homogeneous and are non-sterile dosage forms.

Commonly used excipients in solution:

• The vehicle
• co-solvents, e.g. glycerin
• Surfactants or surface-active agents to enhance the solubility of the therapeutic ingredient
• preservatives, e.g., sorbic acid and phenolics
• sweeteners, e.g., saccharin
• viscosity modifiers, e.g., alginic acid
• Antioxidants, e.g., butylated hydroxytoluene
• coloring agents 
• flavors
• Buffers, e.g., citrate Buffer, to regulate the pH of the formulation.

Pharmaceutical Solutions: Advantages

• Individuals like elderly patients, infants may have difficulty swallowing medicines. Solutions help in the ease of administration of therapeutic drugs.
• The dissolved therapeutic agent in the formulation enhances the bioavailability, making the drug immediately available for absorption. 
• The dissolved ingredients do not precipitate within the gastrointestinal tract.
• You can add flavors and sweeteners to mask the bitter therapeutic agents.

Disadvantages

• Solutions are unsuitable pharmaceutical preparations for chemically unstable therapeutic ingredients in water. 
• Therapeutic agents which are less soluble may prohibit the formulation process. Though, you can enhance solubility by adding co-solvents.
• Solutions as oral dosage form are comparatively bulky preparations. Distributing such solutions is always critical and expensive.  

Read more about the active pharmaceutical ingredients and excipients. 

Factors affecting the solubility

Let us understand how the physicochemical properties affect the solubility of therapeutic agents?

• Melting point- as it increases, solubility decreases. Both are inversely related.
• Type of chemical substituent groups: A therapeutic agent’s solubility is directly affected by the type and position of chemical substituent groups. e.g., the hydrophilic group has better solubility than the lipophilic groups.
• pH– The solubilities are pH-dependent for the acid and base nature of drugs.

Formulation methods: How to enhance the solubility of solutions?

1. Appropriate selection of drug salt: as it allows required solubility in the dosage form.
2. Optimization of the formulation’s pH: control the pH and use buffers if required to achieve solubility.
3. Use of co-solvents: in the case of poorly soluble drugs. 
4. Selection of the most suitable solvent system: for the vehicle, water, and the chosen co-solvent.

Types of Pharmaceutical Solutions

1. Pharmaceutical solutions for oral administration- There are three principal types of solution formulations administered orally: oral solutions, oral syrups, and oral elixirs.
2. Miscellaneous oral solutions- other solution-based dosage forms are linctuses and mouthwashes/gargles.
3. Enemas: Enemas are pharmaceutical solutions that are administered rectally and are employed to ensure clearance of the bowel

Let us discuss cough syrup as a pharmaceutical solution, to understand the types and uses. (1)

Types of Cough Syrup