Quality Control of Ointments

Generally ointments are evaluated for the following qualities:

(1) Penetration: 

For assessing the penetration some vary simple experiments have been suggested. Weighed quantities of the ointments are rubbed over definite areas of the skin for a given length of time. Thereafter the unabsorbed ointment is collected from the skin and weighed. The difference between the two weights roughly represents the amount absorbed.

(2) Rate of release of medicaments: 

To assess rate of release of a medicament small amount of the ointment can be placed on the surface of nutrient agar contained in a Petri dish or alternately in a small cup cut in the agar surface. If the medicament is bactericidal the agar plate is previously seeded with a suitable organism like S. aureus. After a suitable period of incubation the zone of inhibition is measured and correlated with the rate of release. The rate of release of salicylic acid is ascertained by incorporating an iron salt in the agar and measuring the colored zone around the spot where ointment was applied.

(3) Absorption of medicaments into blood stream: 

The diadermatic ointments should be evaluated for the rate of absorption of drug into the blood stream. This test can be run in vivo only. Definite amounts of ointments should be rubbed through the skin under standard conditions and medicaments estimated in the blood plasma or urine.

(4) Irritant effect: 

In general no ointment should possess irritant effect on the skin or mucous membranes. The tests for irritancy can be carried out on the skin and eyes of rabbits or the skin of human beings. The irritant effect can also be judged to a certain extent by injecting the ointment into thigh muscles and under the abdominal skin in rats. Reactions are noted at intervals of 24, 48, 72 and 96 hours. Lesions on cornea, iris, conjuctiva are used for judging the irritancy to the eyes. Presence of patches on the skin within 2 weeks indicate irritancy to skin.