Long-course antibiotics — patient guide

The five things to lock in at initiation.

1. Upright swallowing. Take the tablet sitting or standing, with a full glass of water, and stay upright for at least 30 minutes. This is not optional. Doxycycline can lodge in the lower oesophagus and cause a localised ulcer (pill oesophagitis) — painful, slow to heal, and entirely preventable by upright swallowing with water. Do not take the dose immediately before lying down to sleep.
2. Photoprotection — SPF50+ daily, hat, peak-UV avoidance. Doxycycline causes photosensitivity — severe sunburn from minimal exposure. Daily broad-spectrum SPF50+, a hat, long sleeves where practical, and avoid the 10am-4pm UV peak during AU summer. This is a core counselling point at initiation.
3. Separation from iron, calcium, magnesium, zinc, and antacids by 2-3 hours. Doxycycline binds (chelates) with these metals in the gut and neither gets absorbed. Includes calcium-fortified plant milks, multivitamins, bismuth (Pepto-Bismol), and sucralfate. Timing fix, not an avoidance issue.
4. Pregnancy and children. Avoid doxycycline after 18 weeks of pregnancy (tooth and bone development) and in children under 12 (tooth staining and enamel hypoplasia). If pregnancy is being planned, discuss alternatives with the prescribing GP.
5. The 3-6 month duration ceiling. Australasian College of Dermatologists pathways limit doxycycline courses for acne to 3-6 months. The goal is to step down to topical therapy alone — the topical retinoid plus benzoyl peroxide carries the longer-term load. If the conversation has drifted into “just stay on it”, reopen the conversation.

The 40 mg modified-release dose for rosacea. This is a sub-antimicrobial anti-inflammatory dose, developed specifically to retain the anti-inflammatory action in papulopustular and ocular rosacea while reducing resistance pressure. The debate about whether the resistance benefit is real is ongoing, but the dose is on Australasian College of Dermatologists rosacea pathways and is the lower-risk choice in long-term rosacea management.

Watch out for. New persistent severe headache or visual disturbance — doxycycline has a rare association with intracranial hypertension. Co-prescribing with isotretinoin (for severe acne) increases that risk and is generally avoided.

Dose. 150 mg taken at night, typically for 6-12 months and then reassess.

The big safety signal. Trimethoprim has an amiloride-like effect at the distal renal tubule, which means it can raise blood potassium. The risk multiplies if you are also on an ACE inhibitor (such as perindopril or ramipril), an angiotensin receptor blocker (such as irbesartan or candesartan), spironolactone, eplerenone, potassium supplements, or if you have reduced kidney function. The standard monitoring pattern is a baseline potassium and creatinine, then a recheck at 1 month, then every 3-6 months — especially if you are over 65 or on RAS-blocking medication.

Pregnancy. Trimethoprim is a mild folate antagonist. Avoid in the first trimester of pregnancy because of the neural tube defect signal. If you are planning a pregnancy, switch to a different prophylactic option.

Resistance pattern. AURA national report data shows rising community resistance of E. coli (the bug responsible for most UTIs) to trimethoprim — above 25% in some AU regions. Local resistance data, and your own pattern of breakthrough infections, should inform whether trimethoprim remains a sensible choice or whether nitrofurantoin or methenamine is a better fit.

Drug interactions to flag. Trimethoprim plus methotrexate (additive antifolate toxicity — avoid combination). Trimethoprim plus warfarin or phenytoin (levels can rise — increase monitoring).

Dose. 50 mg taken at night with food, typically for 6-12 months and then reassess. Food improves both tolerability and absorption.

The two safety signals worth knowing.

1. Pulmonary. Nitrofurantoin has two distinct lung patterns at long-term use. Acute hypersensitivity pneumonitis can develop within the first weeks (cough, fever, breathlessness, sometimes rash). Chronic interstitial pneumonitis and pulmonary fibrosis is the longer-term signal — months to years of exposure, and presenting as a slow-onset dry cough or progressive breathlessness. The action for either pattern is the same: any new persistent dry cough, unexplained breathlessness, or fatigue while on long-term nitrofurantoin warrants stopping the tablet and presenting for review with a chest X-ray. This is not over-cautious — it is the published red flag.
2. Hepatic. Chronic active hepatitis is a recognised signal at long-term use. Baseline liver function tests and a repeat at 3-6 months are the standard monitoring rhythm. Report jaundice, dark urine, or persistent right upper abdominal pain.

Kidney function. Nitrofurantoin only achieves therapeutic levels in urine — it does not treat pyelonephritis or systemic infection. It is avoided if your eGFR is below 30 mL/min/1.73m² (the urinary concentration is inadequate and the toxicity risk rises). Use cautiously in the 30-45 range; your GP will weigh this case by case.

Pregnancy. Generally acceptable through most of pregnancy but avoided at term (around 38 weeks onwards) because of a neonatal haemolysis signal in G6PD deficiency.

Drug interactions to flag. Magnesium-containing antacids reduce nitrofurantoin absorption — separate doses. Probenecid raises nitrofurantoin blood levels and reduces urinary concentration (so the UTI effect drops). Quinolone antibiotics (such as ciprofloxacin) can have antagonistic effects.

The mechanism. Methenamine hippurate is not an antibiotic. In acidic urine (pH below about 6.0) it hydrolyses to formaldehyde, which has broad bactericidal action across the bacteria that cause urinary tract infections. Because the mechanism is chemical rather than antibiotic, methenamine does not contribute to antimicrobial resistance — this is the major stewardship advantage that the Cochrane review and the ALTAR trial (BMJ 2022) sit behind.

Dose. 1 g twice daily.

What defeats the mechanism. Anything that alkalinises the urine. That includes Ural and Citravescent sachets, sodium bicarbonate, acetazolamide, and some antacid regimens. If you are already using urinary alkalinisers for symptom relief during cystitis flares, the conversation with your GP is about choosing one strategy or the other — not both.

When methenamine fits. Patients who want prophylactic benefit without the resistance trade-off. Patients with rising trimethoprim resistance in their local area. Patients who have had problems with nitrofurantoin tolerability or are out of range for it on eGFR. Patients who have completed a long course of antibiotic prophylaxis and want to step down to a non-antibiotic maintenance option.

Where methenamine does not fit. Severe renal impairment, severe hepatic impairment, or metabolic acidosis. Pyelonephritis (it works in the bladder, not the upper urinary tract). The other thing to know is that sulfonamides (like sulfamethoxazole) combined with methenamine in acidic urine can crystallise — avoid the combination.

Tolerability. Generally good. Some patients report mild GI upset or bladder irritation, which usually settles. Dietary acidification (cranberry juice or vitamin C) is sometimes added to support the acidic-urine requirement, though the evidence for that adjunct is modest.

Framing. This indication is specialist-initiated — the prescribing decision comes from your respiratory physician, not from primary level general practice. The role of the GP on this page is to support the regimen, monitor for the known signals, and coordinate medication review.

Azithromycin dose. 500 mg three times weekly (typically Monday, Wednesday, Friday) for non-CF bronchiectasis (BLESS regimen) or severe COPD with frequent exacerbations.

The safety stack.

1. QT prolongation. Baseline ECG before initiation. Review the rest of the medication list for other QT-prolonging drugs — citalopram, escitalopram, ondansetron, amiodarone, sotalol, methadone, fluoroquinolones, some antipsychotics. Report syncope or palpitations promptly.
2. CYP3A4 interactions. Macrolides slow the breakdown of a long list of drugs through the CYP3A4 enzyme. Statins (especially simvastatin and atorvastatin) — myopathy risk. Calcium channel blockers (verapamil, diltiazem, amlodipine) — hypotension risk. Warfarin — INR shifts. Calcineurin inhibitors (ciclosporin, tacrolimus) — toxic level rise. Erythromycin has a higher CYP3A4 load than azithromycin, but the azithromycin signal is real and not zero. A pharmacist medication review at initiation is the lowest-friction safety net.
3. Hearing changes. Sensorineural hearing loss and tinnitus signals at long-term high-dose macrolide use. Report any new hearing change or persistent tinnitus.
4. Hepatotoxicity. Baseline and periodic liver function tests on long-course macrolides.
5. Macrolide resistance in nasopharyngeal flora. Established within weeks of starting and persistent. Part of the risk-benefit conversation with your respiratory physician.

Erythromycin specifics. The older macrolide alternative when azithromycin is not tolerated. Higher CYP3A4 interaction surface, more GI side effects (erythromycin acts as a motilin agonist and stimulates gut motility), higher QT load. Same specialist-initiated framing.

Consistency matters. Missed doses reduce the prophylactic effect — the regimen works through sustained tissue levels, not peak action.

Long-course antibiotic exposure is a major risk factor for Clostridioides difficile colitis — an overgrowth of an opportunistic bacterium in the gut that produces a toxin and causes severe inflammatory diarrhoea.

The threshold to act. New persistent watery diarrhoea — 3 or more loose stools per day — that develops while on, or within 8 weeks of finishing, a long course of antibiotics is the published red flag. The action is two-step: stop the antibiotic, and seek same-day GP review for stool C. difficile testing.

Severity markers that need urgent ED review rather than a GP appointment. Severe abdominal pain, fever, blood in the stool, signs of dehydration (light-headedness, low urine output), or significant systemic illness. C. difficile colitis can become a surgical emergency in its severe form.

Sub-antimicrobial doxycycline. The 40 mg modified-release dose for rosacea is below the threshold normally associated with C. difficile selection. The threshold message still applies to standard 50-100 mg doxycycline courses, all UTI prophylactic agents, and long-course macrolides.

Vaginal and oral thrush. Less serious but very common during long courses. Anticipate it, have a treatment plan (clotrimazole pessary, oral fluconazole) ready, and message the prescribing GP if the pattern is repetitive.

Always tell the GP and pharmacist about:

• All prescription medicines (especially blood thinners, blood pressure medicines, antidepressants, heart-rhythm drugs, transplant medicines, antiseizure drugs)
• Over-the-counter medicines (antacids, anti-inflammatories, Pepto-Bismol, sucralfate)
• Supplements (iron, calcium, magnesium, zinc, multivitamins, herbal preparations, methylated folate)
• Urinary alkalinisers (Ural, Citravescent) if you are starting methenamine

The key interaction families.

• Doxycycline plus iron / calcium / magnesium / zinc / antacids / bismuth / sucralfate — chelation. Separate by 2-3 hours.
• Doxycycline plus warfarin — increased anticoagulant effect. Increase INR monitoring.
• Doxycycline plus oral retinoids (isotretinoin, acitretin) — additive intracranial hypertension risk. Generally not co-prescribed.
• Trimethoprim plus ACE inhibitors / ARBs / spironolactone / eplerenone / potassium supplements / potassium-based salt substitutes (LoSalt, NoSalt) — additive hyperkalaemia. Monitor potassium.
• Trimethoprim plus methotrexate — antifolate toxicity. Avoid.
• Nitrofurantoin plus magnesium-containing antacids — reduced absorption. Separate doses.
• Methenamine plus urinary alkalinisers (Ural, Citravescent, sodium bicarbonate) — defeats the mechanism. Do not combine.
• Methenamine plus sulfonamides — crystalluria risk in acidic urine. Avoid.
• Macrolides plus statins (especially simvastatin, atorvastatin) — myopathy risk via CYP3A4. Pharmacist review.
• Macrolides plus calcium channel blockers (verapamil, diltiazem, amlodipine) — hypotension via CYP3A4. Pharmacist review.
• Macrolides plus warfarin — INR shifts. Increase monitoring.
• Macrolides plus other QT-prolonging drugs (citalopram, escitalopram, ondansetron, amiodarone, sotalol, methadone, fluoroquinolones) — additive QT prolongation. ECG at baseline.
• Macrolides plus calcineurin inhibitors (ciclosporin, tacrolimus) — toxic level rise via CYP3A4. Specialist input.

The combined oral contraceptive pill — modern AU consensus. The older blanket “antibiotics reduce pill effectiveness” advice has softened in current AU practice. The established interaction is between rifampicin or rifabutin (strong CYP3A4 inducers) and the combined pill. For routine long-course doxycycline, trimethoprim, nitrofurantoin, methenamine, azithromycin, or erythromycin, there is no automatic requirement for additional contraception. If pregnancy avoidance is critical, barrier backup during the first 2 weeks of a new course is a reasonable belt-and-braces position — worth discussing with the prescribing GP.

Alcohol. Avoid on extended courses. Additive hepatotoxicity (nitrofurantoin, macrolides), GI irritation, microbiome stress, and impaired infection clearance. Not a strict pharmacological interaction with doxycycline but the lifestyle direction stands.

The monitoring pattern depends on the agent.

Doxycycline (acne, rosacea). Generally minimal — baseline if not done recently. No routine ongoing bloods are required for a standard 3-6 month dermatology course. The monitoring that matters is clinical: skin response, photoprotection adherence, GI tolerability.

Trimethoprim (UTI prophylaxis). Baseline urea, creatinine, electrolytes (potassium). Recheck at 1 month. Then every 3-6 months — more frequent if you are over 65, on RAS-blocking medication (ACE inhibitor, ARB, spironolactone, eplerenone), or have CKD.

Nitrofurantoin (UTI prophylaxis). Baseline liver function tests and full blood count. Repeat every 3-6 months. Clinical check at every review for new respiratory symptoms.

Methenamine (UTI prophylaxis). Baseline renal and hepatic function. Repeat annually. Urinary pH check (informal) is sometimes useful — if pH is consistently above 6 the mechanism is not engaging.

Azithromycin / erythromycin long-course (chronic respiratory disease). Baseline ECG. Baseline liver function tests and electrolytes. Repeat liver function 3-6 monthly. Audiometry at baseline and on symptoms. Coordinated by the respiratory physician.

Universal review point. The 6-month review of indication, duration, and the question of whether to continue is the baseline standard regardless of agent.

Stopping the course. Generally the course is stopped on the planned end-date, not tapered. Sudden cessation does not cause a withdrawal pattern with antibiotics in the way it can with some other long-course medicines. The exceptions are: a new adverse signal (C. difficile pattern, pulmonary symptoms on nitrofurantoin, hyperkalaemia on trimethoprim, hearing change on azithromycin) where the antibiotic is stopped immediately and a different approach is found, and specialist-initiated regimens where the respiratory physician sets the stop date.

The 8-12 week gut recovery period. The microbiome takes time to rebuild after a long course. A deliberate recovery protocol:

• Fibre diversity — target 30 or more different plant foods per week (vegetables, fruits, legumes, nuts, seeds, whole grains, herbs, spices). Diversity of plant species matters more than total fibre grams.
• Fermented foods daily — yoghurt with live cultures, kefir, sauerkraut, kimchi, tempeh, miso. Whole-food sources have an ecological advantage over isolated supplements.
• Polyphenol-rich foods — berries, extra-virgin olive oil, dark chocolate (above 70% cocoa), green tea, herbs and spices.
• Reduce ultra-processed foods — additives and emulsifiers interfere with mucosal barrier function.
• Optional probiotic supplement — multi-strain formulations including Lactobacillus rhamnosus GG and Saccharomyces boulardii for 4-8 weeks post-completion, taken away from any remaining medications.

The science here is still evolving — effect sizes are debated, and individual responses vary. The pragmatic direction is supportive of deliberate recovery rather than passive return-to-baseline.

Each agent has a different profile — this is one of the places where the choice of long-course agent matters a lot in a woman of reproductive age.

• Doxycycline — avoid after 18 weeks of pregnancy (tooth and bone development). Avoid during breastfeeding if the infant is under 12 months where possible; case-by-case otherwise.
• Trimethoprim — avoid first trimester (folate antagonist, neural tube defect signal). Acceptable later in pregnancy with folate supplementation. Compatible with breastfeeding.
• Nitrofurantoin — generally acceptable through most of pregnancy but avoid at term (around 38 weeks onwards) because of neonatal haemolysis in G6PD deficiency. Compatible with breastfeeding except in G6PD-deficient infants.
• Methenamine — generally acceptable in pregnancy and breastfeeding; specific data is limited so case-by-case.
• Azithromycin — generally acceptable in pregnancy. Compatible with breastfeeding.
• Erythromycin — generally acceptable; the estolate salt has been associated with maternal hepatotoxicity and is avoided in pregnancy.

If you are planning a pregnancy and are on a long course of any antibiotic, discuss timing and agent choice with the prescribing GP before trying to conceive. If you discover you are pregnant while on a long course, contact the prescribing GP within the next day or two — do not panic, but do not let the conversation wait.

Most of the long-course antibiotics on this page are on the PBS. From 1 January 2026, the PBS co-payment is:

• General patient: up to $25.00 per script
• Concession card holder: up to $7.70 per script

Generic versions cost the same as branded ones at PBS pricing and work the same. Methenamine hippurate (Hiprex) is on the PBS for recurrent UTI prophylaxis. Long-course azithromycin and erythromycin for non-CF bronchiectasis or COPD are typically prescribed under PBS authority — the respiratory physician will arrange the authority script.

Doxycycline beyond 6 months in acne. Confirm the current PBS authority pathway with the prescribing GP, as the criteria have shifted over recent PBS editions.

Confirm exact pricing with the pharmacist — they can show the cheapest available option for the script.