Some drugs can only be administered systemically. Because of the extensive blood supply to the lids, orbit, and posterior segment, drugs given via local injection can dissipate rapidly. In these cases, venous or oral administration can be highly effective and is often the route of choice. Drugs given in these ways have the potential to affect the body's entire system. Side effects and drug interactions increase markedly when drugs are administered systemically.
Intravenous injections (IVs) release the drug directly into the blood stream. IVs are routinely used during intraocular surgery. In cases of endophthalmitis, antibiotics are given through this route. During an angle-closure glaucoma attack, IV drugs are used to help lower the intraocular pressure (IOP). IVs are also used in administering sodium fluorescein for the photographic diagnosis of retinal disorders.
Orally administered medications are absorbed through the stomach and/or the intestines. They then make their way via the blood stream to where they are needed. Oral medications are used in instances where topical medications are not effective or when it is suspected or known that an eye condition has a systemic cause. An example of the latter is where oral antibiotics are used to treat serious eyelid infections or where oral steroids are given for the treatment of Graves' disease. Oral medications are also used in some cases of severe glaucoma.
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