Stability of medicines

What is the role of the pharmacist in stability of medicines?

design, maintenance of stability, advise on medicine use and judgement on implications

What is stability?

ability to retain original characteristics during manufacture to use.

What changes occur in physical instability?

• bioavailability (solubility/evaporation)

• medicine form (separation)

• patient acceptability ( viscosity or colour)

When may suspensions become physically unstable?

due to settling or caking.

When may emulsions become unstable?

separation of dispersed phase or cracking.

What comes under incompatibility?

pharmacological, pharmacokinetic, interactions and physicochemical

What are the physicochemical effects of instability?

solubility, eg diazepam has poor solubility so injections have to be modified.

What is modified if a drug is poorly water soluble?

given as sodium or potassium which are water soluble

What does it mean if pH increases?

more drug to HA, increased solubility.

What do we do if a basic drug is poorly water soluble?

given as hydrochloride to increase pH.

What happens when esters are degraded?

turn to carboxylic acids.

What happens when amides degrade?

less reactive due to basic leaving group

What happens when thioesters are degraded?

not stabilised by resonance.

What is oxidation?

loss of H, gain of O.

How can we minimise oxidation?

prevent initiation by protecting from light and peroxides.

Stop propagation by excluding oxygen, add antioxidant.