The methods applied in compounding extemporaneous formulations found in prescriptions arriving at our pharmacies may vary. At the same time, the interactions among the ingredients in the formula (whether they are incompatible or not), their order of mixing, and their quantities determine the maximum benefit the patient will derive from that formula.
If you wish, let us examine together the importance of the method in compounding extemporaneous prescriptions with an example.
Rp
a) Coal tar …………………… 1.50 g
b) Zinc oxide ………………..
c) Starch …………………… / aa 3.00 g
d) Petrolatum (Vaseline) ………………………… 30.00 g
M.ft. Mist. (Make a mixture)
Sig. : Us.Ex. (For external use)
It might seem possible to prepare this formula in the ways specified below:
- a+b+c+d or;
- it can be prepared as b+d+a+c.
However, neither method will be correct.
The ideal compounding method should be as follows:
- A: a+c+1/4 d ………………… in a capsule/evaporating dish
B: b+3/4 d ……………………… in a mortar
We should add A gradually onto B and mix thoroughly. Meanwhile, we must remember that heating the 3/4 portion of petrolatum to be added to the zinc oxide in a mortar is important in terms of formula homogeneity and the quality of the preparation.
Tars, especially coal tar, do not mix completely with petrolatum; starch and polysorbate 80 can be good aids in this regard. Generally, in coal tar ointments containing zinc oxide, the zinc oxide is first mixed thoroughly with the base, and then the tar mixed with the adjuvant is added. When weighing tar, it is very practical to first use a tared greaseproof paper coated with petrolatum. Coal tar can also mix with washable bases. Solid substances must definitely be brought out of their crystalline state, either thoroughly levigated with liquid petrolatum (mineral oil), levigated in the base, or incorporated into the base dissolved in a solvent.
It should not be forgotten that the physical appearance of such formulas—which might always be compounded using different methods by pharmacists—can be easily distinguished by patients, and compounding these formulas using different methods in different pharmacies may raise doubts and questions for patients.
Wishing you a good week…
Exp. Pharm. Ahmet Nezihi Pekcan
Pekcan Pharmacy – Konya
[email protected]
Tel: (332) 3520657
http://www.majistralformul.com/
