Penicillin-Class Antibiotic (Amidinopenicillin) Pregnancy: eMC §4.6: a large amount of data on pregnant women (more than 40 000 pregnancy outcomes) indicates no malformative nor feto/neonatal toxicity. 'Pivmecillinam can be used for treatment of acute uncomplicated cystitis during pregnancy, if clinically needed.' Intake shortly before delivery may cause a false-positive newborn screening test simulating isovaleric acidaemia. Breast-feeding: mecillinam is excreted in human milk, but at therapeutic doses no effects on the breast-fed newborn/infant are anticipated — 'Pivmecillinam can be used during breast-feeding.' Fertility: no clinical studies; a pre-clinical study showed no effect on fertility in rats.

Pivmecillinam

Brand names: Selexid

Pivmecillinam is an orally administered prodrug of mecillinam, a penicillin-class antibacterial, used principally for the treatment of acute uncomplicated lower urinary tract infection.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Acute uncomplicated cystitis: 72-hour course of 2 tablets immediately, followed by 1 tablet 3 times daily, to a total of 10 tablets (the SPC product is Pivmecillinam 200 mg film-coated tablets)

Route: Oral — 'The tablet must be taken with at least half a glass of fluid and preferably taken with or immediately after a meal' (risk of oesophageal ulceration)

Frequency: Initial 2-tablet dose, then 1 tablet three times daily for a 72-hour course

SOURCE: eMC UK SPC 'Pivmecillinam 200mg film-coated tablets' §4.2, quoted verbatim. The SPC expresses the cystitis regimen in TABLETS, not milligrams — the product is a 200 mg film-coated tablet. OTHER ADULT INDICATIONS (verbatim §4.2): chronic or recurrent bacteriuria — '2 tablets three to four times daily'; salmonellosis — enteric fever '1.2-2.4 g daily for 14 days', Salmonella carriers '1.2-2.4 g daily for 2-4 weeks'. PREGNANT WOMEN: acute cystitis — '7 day course of 2 tablets immediately followed by 1 tablet 3 times daily to a total of 22 tablets'. ELDERLY: 'Renal excretion of mecillinam is delayed in the elderly, but significant accumulation of the drug is not likely at the recommended adult dosage.' CHILDREN WEIGHING MORE THAN 40 kg: the adult regimens above apply. US CROSS-CHECK (differs from the UK regimen — do not merge): the US label for PIVYA states 'one 185 mg tablet orally 3 times a day for 3 to 7 days as clinically indicated', with or without food; each PIVYA tablet contains 185 mg pivmecillinam equivalent to 200 mg pivmecillinam hydrochloride. CARNITINE: the SPC warns against long-term or frequently repeated treatment because of carnitine depletion (muscle aches, fatigue, confusion). NEONATAL SCREENING: intake shortly before delivery may cause a false-positive newborn test for isovaleric acidaemia. §4.8 was truncated at the source-fetch limit.

Paediatric dose

Route: Oral — with at least half a glass of fluid, preferably with or immediately after a meal

Frequency: Daily dose given in 3 to 4 divided doses

eMC §4.2, children weighing LESS than 40 kg: urinary tract infections '20-40 mg/kg body weight, daily, in 3 to 4 divided doses'; salmonellosis '30-60 mg/kg body weight, daily, in 3 to 4 divided doses'. dosePerKg is left null because the SPC states a RANGE and the range differs by indication — carrying a single number would risk the wrong figure being applied. Children weighing MORE than 40 kg follow the adult regimens (acute uncomplicated cystitis: 2 tablets immediately then 1 tablet three times daily over 72 hours, total 10 tablets). No paediatric maximum dose is stated in this SPC. §4.8: frequency, type and severity of adverse reactions in children are expected to be the same as in adults, based on limited data. The US label states safety and effectiveness have not been established in paediatric patients, and reports symptomatic hypocarnitinaemia in paediatric patients on long-term pivmecillinam outside the US (irritability, altered mental status, fatigue, muscle weakness, vomiting). Verify all under-18 dosing against a children's formulary.

Dose adjustments

Renal

No numeric renal dose adjustment is stated in the fetched SPC. §4.4 warns: 'As with other antibiotics which are excreted mainly by the kidneys, raised blood levels of mecillinam may occur if repeated doses are given to patients with impaired renal function.' The US label additionally advises considering alternative antibacterials in patients with significant renal impairment because of carnitine-depletion risk. Clinician to confirm renal dosing from current prescribing references.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to the active substance or to any of the excipients
Hypersensitivity to penicillins and/or cephalosporins
Any conditions resulting in impaired transit through the oesophagus
Genetic metabolism anomalies known to lead to severe carnitine deficiency, such as carnitine transporter defect, methylmalonic aciduria or propionic acidaemia
(US label additionally lists acute porphyria as a contraindication; the UK SPC places porphyria under §4.4 cautions — 'Pivmecillinam should be used with caution in patients with porphyria since pivmecillinam has been associated with acute attacks of porphyria')

Side effects

Diarrhoea and nausea (common) — the most frequently reported adverse reactions
Vulvovaginal mycotic infection (common); Clostridium difficile colitis (uncommon), with fatal pseudomembranous colitis reported
Vomiting, abdominal pain, dyspepsia, oesophageal ulcer, oesophagitis and mouth ulceration (uncommon)
Headache, dizziness and vertigo (uncommon); fatigue (uncommon); decreased carnitine (uncommon)
Anaphylactic reaction and thrombocytopenia (uncommon); anaphylactic shock (not known); rash, urticaria and pruritus (uncommon); severe cutaneous adverse reactions including AGEP, DRESS, Stevens-Johnson syndrome and toxic epidermal necrolysis, and angioedema (not known)

Interactions

Probenecid — reduces the excretion of penicillins and hence increases the blood level of the antibiotic
Methotrexate — clearance from the body can be reduced by concurrent use of penicillins
Valproic acid, valproate or other medication liberating pivalic acid — concurrent treatment should be avoided due to increased risk of carnitine depletion
Bacteriostatic agents, for instance erythromycin and tetracyclines — may hinder the bactericidal effect of penicillins
Newborn screening tests — treatment of a pregnant patient shortly before delivery may cause a false-positive test for isovaleric acidaemia in the newborn

Clinical monograph

How it works

After absorption it is hydrolysed to mecillinam, which binds penicillin-binding protein 2 and inhibits bacterial cell wall synthesis, with particular activity against Gram-negative organisms such as Escherichia coli.

Prescribing in practice

It is contraindicated in patients with a history of penicillin hypersensitivity owing to the risk of cross-reactivity, including anaphylaxis.
It should be taken with food and an adequate volume of fluid while sitting or standing upright to reduce the risk of oesophageal irritation.
Pivmecillinam is associated with a risk of carnitine depletion and is not recommended where carnitine deficiency is a concern or for prolonged or repeated courses without specialist advice.

Monitoring

Routine laboratory monitoring is not generally required for short courses; review clinical response and reassess if symptoms fail to resolve.

Counselling the patient

Take each dose with food and plenty of water, and do not lie down immediately afterwards.
Complete the full course even if symptoms improve, and report any rash, swelling or difficulty breathing urgently.

Evidence & guidelines

It is recommended by NICE as a first-line option for uncomplicated urinary tract infection; consult current guidance for local antimicrobial recommendations.

Reference: NICE NG109 (UTI in adults); PHE Antimicrobial Prescribing Guidelines; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways