Incompatibility of Erythromycin and Hydroquinone

Erythromycin requires a pH of 8 for optimal stability. Hydroquinone contains two phenol groups that will be ionized at this pH, leading to instability. Combining these two substances in a single cream is therefore not ideal.

If no discoloration occurs in an anhydrous medium, the result is an ointment rather than a cream. For this reason, separate preparations must be made for each of these substances.

Preparation

For erythromycin, it is recommended to use the erythromycin gel described in the TMF (Therapeutic Magistral Formulary).

For the hydroquinone cream, the following composition can be used:

• Hydroquinone: 4 g
• Glycerin: 25 g
• Na2EDTA: 50 mg
• Na metabisulfite: 500 mg
• Purified water: 3 g
• Hydrophilic anionic cream: 67.5 g

Preparation Method for Hydroquinone Cream

1. Dissolve hydroquinone in the glycerin by heating to approximately 60 °C.
2. Dissolve Na2EDTA and Na metabisulfite in 3 ml of water.
3. Mix this aqueous solution with the hydrophilic anionic cream.
4. Mix the hydroquinone solution with the hydrophilic anionic cream.

Shelf Life and Storage Conditions

Both preparations have a shelf life of at least 2 months. The hydroquinone cream must not be stored in the refrigerator.