Eye treatment: drug administration

Drug administration to the eye

Drugs are most commonly administered to the eye by topical application as drops or ointment.

Eye drops are instilled into the pocket formed by gently pulling down the lower eyelid and keeping the eye closed for as long as possible after application. 

Eye ointment is applied similarly and blinking helps it to spread.

If two different eye drop preparations are used, leave an interval of at least 5 minutes between the two or dilution and overflow can occur.

Systemic effects can arise from absorption of drugs into the general circulation from conjunctival vessels or from the nasal mucosa. Pressure on the bridge of the nose reduces nasolacrimal drainage and decreases systemic absorption from the nasal mucosa.

How many eye drops to prescribe

Although the size of a drop from a dropper bottle varies with viscosity, dropper design and patient technique, it is normally around 35 to 50 microlitres.

An average eye brimming with fluid can only hold about 30 microlitres of fluid.

Ophthalmology experts consider that a dose of one drop will be more than sufficient: a second drop doubles the cost and will either wash out the first drop or increase the risk of systemic toxicity.

Control of microbial contamination

The manufacturer's expiry date can be used for all eye drops (and ear and nose drops), except for: 

• Inpatients with eye infection - separate bottles for each eye – but still a 28-day expiry for preserved eye drops or 7 days for eye drops without preservative
• Inpatients post-eye surgery or admitted under the eye team – these patients usually have preservative-free single-use eye drops anyway, but all other drops 7-day expiry

Preservative-free eye drops are often in single-use doses, and these should be discarded after use.