Oral Medications

Concise summaries of systemic oral medications relevant to optometric practice in Singapore, including practical guidance on pharmacology, dosing, patient selection, and monitoring for antivirals, antihistamines, antibiotics, carbonic anhydrase inhibitors, corticosteroids, and nutritional supplements.

Last updated: March 2026

What are oral medications in eye care?

Oral (systemic) medications play an important adjunctive or primary role in the management of a range of ocular and periocular conditions encountered in optometric practice. Unlike topical eye drops, oral agents achieve systemic bioavailability — enabling therapeutic drug concentrations in posterior segment tissues, orbital structures, and intraocular compartments that topical preparations cannot reliably reach. They are indicated when topical therapy is insufficient, when systemic disease underlies the ocular presentation, or when emergency IOP reduction is required pending specialist review.

In Singapore, oral ocular-relevant medications span all three HSA regulatory categories — from over-the-counter omega-3 supplements and pharmacy-only antihistamines to Prescription-Only Medicines (POM) such as oral antivirals, doxycycline, systemic corticosteroids, and acetazolamide. Singapore optometrists must understand the pharmacology and clinical context of these agents in order to appropriately counsel patients, recognise adverse effects, and refer when escalation is required.^[1,2]

Drug classes

Oral antivirals — nucleoside analogues (aciclovir, valaciclovir) used systemically for herpes simplex keratitis, herpes zoster ophthalmicus (HZO), and long-term HSV suppression; all POM.
Oral antihistamines — second-generation H1-blockers (cetirizine, loratadine, fexofenadine) as adjuncts for allergic conjunctivitis with systemic atopy; available OTC or as Pharmacy medicine.
Oral antibiotics — tetracyclines (doxycycline, azithromycin) for meibomian gland dysfunction (MGD), ocular rosacea, and periocular bacterial disease; POM.
Systemic carbonic anhydrase inhibitors (CAIs) — acetazolamide (Diamox) for emergency IOP reduction in acute angle-closure glaucoma (AACG); POM; bridge therapy pending ophthalmology referral.
Oral corticosteroids — prednisolone for severe uveitis, optic neuritis, orbital inflammatory disease; strictly POM; prescribing by ophthalmologist or physician only.
Nutritional supplements — omega-3 fatty acids (EPA/DHA) as adjunctive therapy for MGD-associated dry eye; OTC; evidence supports modest improvement in tear lipid quality and TBUT.

Who are they for?

Oral medications in eye care are indicated for a defined set of conditions where systemic therapy is necessary, where topical agents alone are insufficient, or where the ocular presentation is a manifestation of systemic disease. Singapore optometrists will frequently encounter patients already taking these agents prescribed by an ophthalmologist or physician, and must be able to identify relevant adverse effects and drug interactions during routine assessment. Additionally, optometrists may initiate oral omega-3 supplementation and recommend OTC or Pharmacy-class oral antihistamines as part of conservative management of dry eye and allergic conjunctivitis.^[3,4]

Patient profiles and common conditions

Herpes zoster ophthalmicus (HZO)

Reactivation of varicella-zoster virus in the V1 dermatome; oral valaciclovir or aciclovir commenced within 72 hours of rash onset significantly reduces ocular complications including keratitis, uveitis, and postherpetic neuralgia.

Recurrent HSV keratitis

Long-term oral aciclovir or valaciclovir prophylaxis reduces recurrence rate of stromal keratitis by ~40–50%; typically continued for 12 months or more following an acute episode.

Meibomian gland dysfunction (MGD) / ocular rosacea

Oral doxycycline is the standard treatment for moderate-severe MGD and rosacea-associated blepharitis in Singapore; reduces lid margin inflammation, normalises meibum, and improves TBUT over 4–12 weeks.

Allergic conjunctivitis with systemic atopy

Oral second-generation antihistamines are used when nasal, dermatological, or respiratory allergic disease coexists with ocular allergy; most prevalent in Singapore given the high house dust mite burden year-round.

Acute angle-closure glaucoma (AACG)

Oral acetazolamide is used as emergency IOP reduction alongside topical agents while arranging urgent ophthalmology referral; time-critical — irreversible optic nerve damage can occur within hours of acute attack.

Uveitis / optic neuritis

Oral prednisolone is used for severe anterior or intermediate uveitis unresponsive to topical steroids, and for optic neuritis to hasten visual recovery. Strictly ophthalmologist- or neurologist-prescribed; optometrists must refer promptly.

Caution — contact lens wearers

Several oral medications have clinically significant implications for contact lens wearers. Oral doxycycline and isotretinoin cause dry eye and ocular surface changes that may reduce contact lens tolerance — advise patients to reduce wearing time and use preservative-free lubricants. Oral antihistamines (including cetirizine and loratadine) reduce aqueous tear secretion via anticholinergic and antihistaminergic effects, exacerbating lens-related dryness. Patients on oral corticosteroids are at elevated risk of posterior subcapsular cataract and raised IOP — regular slit-lamp and IOP monitoring is essential. In all cases of active ocular infection (HZO, HSV keratitis), contact lens wear must be ceased until the infection has completely resolved and the treating ophthalmologist has cleared the patient to resume.

When should they be used?

Acute presentations — urgent initiation

Oral antivirals for HZO must be commenced within 72 hours of rash onset to meaningfully reduce the risk of ocular complications; efficacy diminishes significantly beyond this window. Any patient presenting with a V1 dermatomal rash, Hutchinson's sign (vesicles on the tip of the nose indicating nasociliary nerve involvement), or ocular symptoms should be referred to an ophthalmologist immediately. In acute angle-closure glaucoma, oral acetazolamide alongside topical IOP-lowering agents is initiated in the emergency setting; Singapore optometrists diagnosing AACG should call for emergency ophthalmology review without delay.

Prophylactic and maintenance use

Long-term oral aciclovir or valaciclovir is used for suppression of recurrent HSV stromal keratitis — a common cause of corneal scarring and visual loss in Singapore. Oral doxycycline is used continuously for 4–12 weeks for MGD and ocular rosacea, often alongside lid hygiene and warm compresses. Oral omega-3 supplementation is ongoing and adjunctive for evaporative dry eye. Oral antihistamines are used seasonally or year-round for perennial allergic conjunctivitis coexisting with systemic atopy, particularly given Singapore's year-round house dust mite burden.

Escalation and referral

Oral corticosteroids for uveitis and optic neuritis, systemic immunosuppressants for chronic uveitis, and high-dose IV methylprednisolone for severe optic neuritis are beyond the optometrist's scope of practice in Singapore — these require specialist initiation and monitoring. Optometrists identifying signs of uveitis (cells and flare, keratic precipitates, posterior synechiae) or acute optic neuritis (reduced VA, RAPD, colour desaturation, periorbital pain on eye movement) must refer to an ophthalmologist or neurologist urgently. Similarly, any patient with suspected AACG or HZO requires same-day emergency referral.

Clinical scenario	Recommended agent	Duration / Action
HZO — within 72 hours of rash	Valaciclovir 1 g TID or aciclovir 800 mg 5× — refer	7 days; urgent ophthalmology review
Recurrent HSV stromal keratitis	Valaciclovir 500 mg OD prophylaxis — refer	12 months minimum; ophthalmologist-prescribed
MGD / ocular rosacea	Doxycycline 100 mg OD — refer for prescription	4–12 weeks; combine with lid hygiene
Allergic conjunctivitis + systemic atopy	Cetirizine 10 mg OD or loratadine 10 mg OD (P)	Seasonal or year-round; combine with topical agent
Acute angle-closure glaucoma	Acetazolamide 500 mg stat — REFER URGENTLY	Emergency bridge; same-day ophthalmology
Anterior uveitis unresponsive to topical steroids	Oral prednisolone — ophthalmologist only	Specialist-directed; taper under supervision
MGD / evaporative dry eye (adjunct)	Omega-3 1–2 g EPA+DHA daily (OTC)	Ongoing; review at 3 months

Where are they available in Singapore?

Oral medications relevant to eye care span all three HSA regulatory categories under the Medicines Act (Cap. 176). Omega-3 supplements are available OTC without restriction; oral antihistamines (cetirizine, loratadine) are Pharmacy medicines available without prescription but under pharmacist supervision; all other oral agents — antivirals, doxycycline, acetazolamide, and systemic corticosteroids — are Prescription-Only Medicines (POM) requiring a valid prescription from a registered medical practitioner.

Scope of practice — Singapore optometrists

Under the Optometrists and Opticians Act (Cap. 213A) and regulations administered by the Optometrists and Opticians Board (OOB), Singapore-registered optometrists are authorised to examine eyes, assess visual function, and prescribe optical appliances. They do not have the authority to prescribe, supply, or administer any therapeutic medication — including all oral POM agents listed on this page. Optometrists may recommend OTC omega-3 supplements and guide patients toward appropriate Pharmacy-class antihistamines, but must refer any patient requiring prescription oral medication to a registered medical practitioner — typically an ophthalmologist or GP with ophthalmic experience.^[5,6]

General Sale (OTC): Omega-3 fatty acid supplements (various brands — Eye q, Blackmores, Nordic Naturals) are classified as health supplements and are available at Guardian, Watsons, Unity, supermarkets, and online without restriction.
Pharmacy medicine (P): Cetirizine (Zyrtec) and loratadine (Clarityne) are Pharmacy-only medicines in Singapore — available without prescription from licensed retail pharmacies under pharmacist supervision. Pharmacists may recommend and supply these agents for allergic symptoms including allergic conjunctivitis.
Prescription-only (POM): Oral antivirals (aciclovir/Zovirax, valaciclovir/Valtrex), doxycycline (Vibramycin), acetazolamide (Diamox), and oral prednisolone all require a valid prescription from a registered medical practitioner. Optometrists detecting conditions warranting these agents must refer without delay.
Hospital formularies: Oral antivirals, acetazolamide, doxycycline, and systemic corticosteroids are standard items on public hospital formularies at eye centres and emergency departments. Prescribing occurs under ophthalmologist supervision; dispensing is in-house or through linked hospital pharmacies.
GP clinics: General practitioners may prescribe oral antivirals, doxycycline, and oral antihistamines for mild-moderate presentations. For ocular complications of HZO, recurrent HSV keratitis, or uveitis, referral to an ophthalmologist is strongly advised.
HSA product verification: Current registration status for all oral agents can be confirmed via the HSA product registration portal (hsa.gov.sg). Verify before recommending or referring for any specific product.^[5]

Why use oral medications in eye care?

Oral systemic agents achieve drug concentrations in anatomical compartments that topical eye drops cannot reliably access — including the posterior corneal stroma, vitreous, retina, choroid, optic nerve, and orbit. For virological conditions such as HZO and HSV keratitis, systemic antiviral exposure is essential to suppress viral replication in the trigeminal ganglion and prevent recurrence or dissemination. For meibomian gland dysfunction, the anti-inflammatory and MMP-inhibitory effects of oral doxycycline are exerted within the meibomian gland epithelium — an effect that topical agents cannot replicate.

Systemic reach

Access to posterior segment, optic nerve, orbital tissues, and trigeminal ganglion — compartments inaccessible to topical drops.

Virological suppression

Oral antivirals suppress HSV/VZV replication at the ganglion level, preventing recurrent keratitis and reducing long-term corneal damage.

Anti-inflammatory at the gland level

Doxycycline exerts anti-inflammatory and MMP-inhibitory effects within meibomian gland tissue — fundamentally altering meibum composition and reducing lid margin inflammation.

Emergency IOP reduction

Oral acetazolamide achieves rapid, substantial IOP reduction (~40–60%) in AACG — essential as a bridge to definitive laser iridotomy.

Oral therapy should always be considered in the context of the full clinical picture. For most conditions, oral agents are used in combination with topical therapy — not as replacements. Omega-3 supplementation is adjunctive and does not replace lid hygiene, warm compresses, or topical lubricants for MGD. Oral antihistamines complement but do not replace topical dual-action agents for allergic conjunctivitis. For conditions requiring POM oral agents, co-management with an ophthalmologist is mandatory.^[7,8]

How to take oral medications — patient guidance

Administration instructions

Take oral medications at the prescribed time and frequency — many agents (particularly antivirals and doxycycline) require strict adherence to dosing intervals to maintain therapeutic blood levels.
Doxycycline must be swallowed with a full glass of water (at least 200 mL) and the patient should remain upright for at least 30 minutes after ingestion to prevent oesophageal ulceration.
Oral antivirals (aciclovir, valaciclovir) should be taken with adequate hydration throughout the day to minimise risk of renal crystalluria, particularly in elderly patients or those with reduced renal function.
Omega-3 supplements are best taken with meals to improve absorption and reduce GI discomfort; enteric-coated formulations reduce fishy aftertaste.
Oral antihistamines (cetirizine, loratadine) may be taken at any time; taking cetirizine at night may minimise any residual drowsiness during waking hours.
Do not stop oral corticosteroids or doxycycline abruptly without medical advice — taper regimens apply for prednisolone; premature discontinuation of doxycycline may result in incomplete treatment of MGD or contribute to antibiotic resistance.

Contact lens guidance

Cease contact lens wear completely during active ocular infection (HZO, HSV keratitis, bacterial conjunctivitis) and for the full duration of antiviral or antibiotic therapy. Resumption requires ophthalmologist clearance.
Oral doxycycline causes photosensitivity — advise patients to use UV-protective sunglasses and avoid prolonged sun exposure, particularly relevant in Singapore's year-round high UV environment.
Oral antihistamines reduce aqueous tear secretion and may worsen contact lens comfort — advise patients to use preservative-free lubricants and reduce lens wearing time if dryness develops.
Patients on long-term oral corticosteroids should have regular slit-lamp examinations to monitor for posterior subcapsular cataract and IOP elevation before resuming or continuing contact lens wear.

Monitoring and red-flag referral

Oral antivirals: Monitor renal function in elderly patients or those on high-dose or long-term regimens; advise adequate hydration; refer if renal impairment develops.
Doxycycline: Review at 4–8 weeks — assess lid margin inflammation, TBUT, and patient tolerance. Advise strict sun protection. Contraindicated in pregnancy and children under 8 years.
Oral corticosteroids: IOP monitoring at every visit; slit-lamp review for posterior subcapsular cataract; blood glucose and blood pressure monitoring if prolonged course. Refer urgently if IOP rises significantly.
Acetazolamide: Monitor serum electrolytes (potassium) and renal function; ensure adequate hydration; watch for paraesthesia and malaise as common side effects of short-term use.
Red flags requiring urgent ophthalmology referral: Sudden loss of vision, new RAPD, optic disc swelling, severe eye pain with nausea/vomiting, corneal clouding, dendritiform corneal lesion, V1 dermatomal rash with ocular involvement, or any deterioration during or after oral therapy.

Common oral medications in Singapore eye care

All agents listed are, or have been, registered with HSA Singapore or are available through registered importers or licensed health supplement channels. Verify current product registration at hsa.gov.sg before recommending or referring for any specific product.

Brand (Generic)	Mechanism	Dosing	Min. Age	Side Effects	Clinical Notes
Zovirax Aciclovir 200 mg tablet POM	Nucleoside analogue — inhibits viral DNA polymerase (HSV, VZV)	HSV keratitis: 400 mg 5×/day × 7–10 days; HZO: 800 mg 5×/day × 7–10 days	Adults; paediatric weight-based	Nausea, headache, renal impairment at high doses	HSV keratitis and HZO; start within 72 h of rash onset; refer to ophthalmologist
Valtrex Valaciclovir 500 mg tablet POM	Prodrug of aciclovir — better oral bioavailability (55% vs 15–30%)	HZO: 1 g TID × 7 days; HSV suppression: 500 mg OD	Adults	Nausea, headache, dizziness	Preferred over aciclovir for HZO; also for long-term HSV stromal keratitis suppression
Vibramycin Doxycycline hyclate 100 mg capsule POM	Tetracycline — inhibits MMP activity, reduces meibomian gland inflammation	MGD/rosacea: 100 mg OD × 4–12 weeks; or 40 mg OD sub-antimicrobial	≥8 years	Photosensitivity, GI upset, oesophageal ulceration; teratogenic	Standard for moderate-severe MGD and ocular rosacea; take upright with full glass of water; avoid in pregnancy
Zyrtec-D Cetirizine 5 mg + Pseudoephedrine HCl 120 mg tablet P	H1-antihistamine (cetirizine) + decongestant (pseudoephedrine)	1 tablet BD; max 2 tablets/day	≥12 years	Insomnia, palpitations, hypertension, raised IOP	Allergic rhinitis with congestion; contraindicated in narrow-angle glaucoma; screen for glaucoma risk
Clarityn-D Loratadine 5 mg + Pseudoephedrine sulfate 120 mg tablet (12-hour) P	H1-antihistamine (loratadine) + decongestant (pseudoephedrine)	1 tablet BD (12-hourly); max 2 tablets/day	≥12 years	Insomnia, palpitations, hypertension, raised IOP	Non-sedating; same IOP/glaucoma contraindications as Zyrtec-D; screen before recommending
Diamox Acetazolamide 250 mg tablet POM	Systemic CAI — reduces aqueous humour secretion ~40–60%	Acute AACG: 500 mg stat, then 250 mg QID	Adults	Paraesthesia, nausea, polyuria, hypokalaemia; avoid in sulphonamide allergy	Emergency IOP reduction in AACG; bridge while arranging urgent ophthalmology referral
Prednisolone 5 mg Prednisolone 5 mg tablet POM	Systemic corticosteroid — anti-inflammatory via glucocorticoid receptor	Optic neuritis: 1 mg/kg/day × 3–5 days with taper; uveitis: specialist-directed	Adults; paediatric weight-based under specialist	Hyperglycaemia, hypertension, peptic ulceration, raised IOP	Optic neuritis, severe uveitis, orbital inflammation; prescribing by ophthalmologist only
Omega 3 Omega-3 Triglycerides OTC	EPA/DHA — modulate eicosanoid synthesis, improve meibomian lipid quality	1–2 g EPA+DHA daily with meals; allow 3 months for full effect	Adults	Fishy aftertaste, GI discomfort, loose stools	Adjunct for MGD-associated dry eye; modest TBUT improvement; OTC; widely available in Singapore

Legend

POMPrescription Only Medicine — Sold or supplied to the public on prescription only.

PPharmacy Medicine — Sold or supplied from any licensed retail pharmacy under pharmacist supervision.

OTCGeneral Sales List (GSL) — Sold or supplied to the public without restriction.

Dosing reflects standard adult doses unless noted. Always refer to the approved product insert for full prescribing information. OD = once daily, BD = twice daily, TID = three times daily, QID = four times daily, PRN = as needed, stat = immediately.

References

[1] Health Sciences Authority Singapore. Medicines Act (Cap. 176). Singapore Statutes Online. Available at: sso.agc.gov.sg (accessed March 2026).
[2] Wilhelmus KR. Antiviral treatment and other therapeutic interventions for herpes simplex virus epithelial keratitis. Cochrane Database Syst Rev. 2015;(1):CD002898. PMID: 25879115.
[3] Herpetic Eye Disease Study Group. Oral acyclovir for herpes simplex virus eye disease: effect on prevention of epithelial keratitis and stromal keratitis. Arch Ophthalmol. 2000;118(8):1030–1036. PMID: 10922192.
[4] Seal DV, Pleyer U (eds). Ocular Infection: Investigation and Treatment in Practice. 2nd edn. London: Informa Healthcare; 2007.
[5] Republic of Singapore. Optometrists and Opticians Act 2007 (No. 16 of 2007). Singapore Statutes Online. Available at: sso.agc.gov.sg/Act/OOA2007 (accessed March 2026).
[6] George PP, Chng OSY, Siow K, et al. Is there scope for expanding the optometrist's scope of practice in Singapore? A survey of optometrists and opticians. Cont Lens Anterior Eye. 2019;42(3):258–264. PMID: 30819628.
[7] Geerling G, Tauber J, Baudouin C, et al. The international workshop on meibomian gland dysfunction: report of the subcommittee on management and treatment of meibomian gland dysfunction. Invest Ophthalmol Vis Sci. 2011;52(4):2050–2064. PMID: 21450919.
[8] Dry Eye Workshop (DEWS II) Management and Therapy Subcommittee. TFOS DEWS II management and therapy report. Ocul Surf. 2017;15(3):575–628. PMID: 28736343.
[9] Bhatt UK, Lagnado R, Dua HS. Aciclovir in the treatment of herpes zoster ophthalmicus. Acta Ophthalmol. 2009;87(4):423–426. PMID: 18788996.
[10] Shtein RM. Post-LASIK dry eye. Expert Rev Ophthalmol. 2011;6(5):575–582. PMID: 22919393.

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