physical and chemical behavior of drug products

Karan sharmaKaran sharma •September 20, 2025

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1. Physical and Chemical Behavior of Drug Products

This part is about how drugs behave in different conditions (solid, liquid, or solution form) and how their stability can be affected.

Solubility – A drug must dissolve before it can act. Poor solubility = poor absorption.
Example: Griseofulvin is poorly soluble, so micronization improves absorption.
Polymorphism – A drug can exist in more than one crystal form. Different forms = different solubility & stability.
Example: Ritonavir tablets failed once because of a new polymorph appearing.
Hygroscopicity – Some drugs absorb moisture from the air (e.g., calcium chloride). This affects stability.
Particle size – Affects dissolution rate, absorption, and even the texture of creams/ointments.
Partition coefficient – Shows how a drug distributes between water and lipid phases (important for crossing membranes).

Hydrolysis – Breakdown due to water.
Example: Aspirin → Salicylic acid + Acetic acid.
Oxidation – Drug reacts with oxygen, losing potency.
Example: Adrenaline (oxidizes to pink/brown color).
Photolysis – Drug degrades in light.
Example: Riboflavin, nitroprusside.
Racemization – Conversion of active enantiomer to inactive one.
Example: L-epinephrine → D-epinephrine (less active).
Polymerization – Small molecules join to form inactive complexes.

👉 Why it matters? If you don’t understand these, you can’t predict drug stability, packaging, or shelf-life.

Here we ask: Why do we make a drug in a specific dosage form (tablet, injection, suspension, cream, etc.) instead of another?

Stability – Some drugs are unstable in liquid form → made as dry powder for reconstitution (e.g., amoxicillin).
Bioavailability – Poorly soluble drugs need formulations like nanosuspensions, solid dispersions, or use of surfactants.
Patient convenience – Syrups for kids, injections for emergencies, transdermal patches for chronic use.
Drug release control – Immediate release vs. controlled/extended release vs. targeted release.
Taste masking – Bitter drugs (e.g., metronidazole) formulated with flavors/coatings.
Compatibility – Excipients are chosen so they don’t chemically react with the drug.

👉 Example: Nitroglycerin is explosive and unstable → formulated in tablets with excipients or transdermal patches for safe, controlled release.

This is the art + science part where pharmacists prepare customized medications when commercial products are not suitable.

Solutions
- Simple solution: Dissolve drug in solvent.
- Syrups, elixirs, tinctures.
- Key concern: Solubility + preservation.
Suspensions
- Insoluble drug dispersed in liquid.
- Must use suspending agents (e.g., methylcellulose).
- Example: Antacid suspensions.
Emulsions
- Oil + water stabilized with emulsifiers.
- Types: O/W (creams, lotions) and W/O (ointments).
- Example: Castor oil emulsion.
Ointments & Creams
- Drug incorporated into a base.
- Levigation (grinding drug with small amount of base/liquid).
Powders
- Simple mixing or geometric dilution for potent drugs.
- Example: Oral rehydration salts (ORS).
Capsules & Tablets (Extemporaneous)
- Filling powders in capsules manually.
- Making small-scale tablets with compression or molding.
Sterile Preparations
- Aseptic techniques for injections, eye drops.
- Filtration, autoclaving, lyophilization.

👉 Modern compounding also includes:

Physical & chemical behavior → tells us how drugs degrade/act.
Rationale of formulation → explains why a drug is made in a certain dosage form.
Compounding techniques → shows how pharmacists prepare customized medicines safely and effectively.

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