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30/10/2025, 00:02 Pharmaceutical Solutions & Sterile Dosage Forms

Drug solubility & bioavailability.

Comfort, safety and activity of the product: Eye irritation leading to increase tear fluid
secretion & Rapid loss of medication.

Non-neutral eye drops may be required for the solubility or stability purpose. However, non-
neutral drugs are irritant.

Buffers used should be of low buffering capacity to enable the tear to bring the pH of the eye
back to physiological range.

Borate & phosphate buffers are the commonly used in ocular formulations.

5. Viscosity

Instillation of foreign body into the eye causes tear secretion and washing out.

Increasing viscosity increases residence time decreasing washing out and increasing absorption
and effect.

Viscosity imparting agents:

They are used to increase the viscosity of solutions and suspensions (retard rate of particle
setting).

They include polyvinyl alcohol and cellulose derivatives such as methyl cellulose (MC) &
hydroxypropyl methyl cellulose (HPMC).

Disadvantage: They produce blurring vision when dry as they form a dry film on the eye lid.

They are used in low concentrations.

Vehicles of ophthalmic preparation:

Ophthalmic aqueous drops use purified water USP or WFI as a solvent.
Oils could be used as vehicles for several topical eye drop products that are extremely sensitive
to moisture.
When oils are used as vehicles in eye drops, they must be of the highest purity.

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