Penicillin (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 the treatment of acute uncomplicated cystitis during pregnancy if clinically needed. Intake shortly before delivery may cause a false-positive newborn screening test for isovaleric acidaemia. Mecillinam is excreted in human milk, but at therapeutic doses no effects on breast-fed newborns/infants are anticipated and pivmecillinam can be used during breast-feeding.

Pivmecillinam hydrochloride

Brand names: Selexid

Used in: Urinary Tract Infection

An oral prodrug of mecillinam, a penicillin-class antibacterial used as a first-line option for 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: 2 tablets immediately, then 1 tablet three times daily, to a total of 10 tablets over a 72-hour course (product = pivmecillinam 200 mg film-coated tablets)

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

Frequency: Three times daily after the initial 2-tablet dose, for 72 hours

eMC §4.2 (Pivmecillinam 200 mg film-coated tablets); doses are expressed by the SPC in tablets, and each tablet contains 200 mg. OTHER ADULT REGIMENS - 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: a 7-day course of 2 tablets immediately followed by 1 tablet three times daily, to a total of 22 tablets. ELDERLY: renal excretion of mecillinam is delayed, but significant accumulation is not likely at the recommended adult dosage. CHILDREN WEIGHING MORE THAN 40 kg: the same regimens as adults (acute uncomplicated cystitis, chronic/recurrent bacteriuria and salmonellosis as above). ADMINISTRATION SAFETY: the tablets must be taken with at least half a glass of fluid because of the risk of oesophageal ulceration, and are contraindicated in any condition causing impaired transit through the oesophagus. Use with caution for long-term or frequently repeated treatment because of possible carnitine depletion (muscle aches, fatigue, confusion); avoid concurrent valproic acid, valproate or other pivalic-acid-liberating medication. During long-term use, routine liver and kidney function tests are advisable.

Paediatric dose

Route: Oral

Frequency: Daily total given in 3 to 4 divided doses

Max: Children weighing more than 40 kg receive the adult regimens (see adult dose)

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 rather than a single value; do not collapse the range to a point value. Children weighing more than 40 kg use the adult regimens. Note the SPC's neonatal-screening warning: pivmecillinam taken shortly before delivery may cause a false-positive newborn screening test for isovaleric acidaemia. Verify against a children's formulary before prescribing.

Dose adjustments

Renal

eMC §4.4: as with other antibiotics excreted mainly by the kidneys, raised blood levels of mecillinam may occur if repeated doses are given to patients with impaired renal function. No numeric dose adjustment is stated in the captured SPC extract.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC — 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 (eMC §4.3)
Hypersensitivity to penicillins and/or cephalosporins (eMC §4.3)
Any condition resulting in impaired transit through the oesophagus (eMC §4.3)
Genetic metabolism anomalies known to lead to severe carnitine deficiency, such as carnitine transporter defect, methylmalonic aciduria or propionic acidaemia (eMC §4.3)

Side effects

Nausea and diarrhoea - the most frequently reported adverse reactions, both common (eMC §4.8)
Vulvovaginal mycotic infection (common); Clostridium difficile colitis and fatal pseudomembranous colitis reported (eMC §4.4 and §4.8)
Oesophageal ulcer, oesophagitis, mouth ulceration, vomiting, abdominal pain and dyspepsia (uncommon)
Rash, urticaria, pruritus (uncommon) and, frequency not known, severe cutaneous adverse reactions including acute generalised exanthematous pustulosis, DRESS, Stevens-Johnson syndrome, toxic epidermal necrolysis and angioedema
Anaphylactic reaction (uncommon) and anaphylactic shock (frequency not known); thrombocytopenia, decreased carnitine, headache, dizziness, vertigo, abnormal hepatic function and fatigue (uncommon)

Interactions

Probenecid (eMC §4.5) - reduces the excretion of penicillins and so increases the blood level of the antibiotic
Methotrexate (eMC §4.5) - clearance of methotrexate can be reduced by concurrent use of penicillins
Valproic acid, valproate or other medication liberating pivalic acid (eMC §4.5) - avoid concurrent treatment because of the increased risk of carnitine depletion
Bacteriostatic agents such as erythromycin and tetracyclines (eMC §4.5) - may hinder the bactericidal effect of penicillins

Clinical monograph

How it works

After absorption it is hydrolysed to mecillinam, which binds penicillin-binding protein 2 to disrupt cell-wall synthesis, giving activity predominantly against Gram-negative urinary pathogens such as Escherichia coli.

Prescribing in practice

Although a penicillin, it should be avoided in patients with penicillin hypersensitivity, so allergy status must be checked before prescribing.
It is intended for lower urinary tract infection and, like other oral pivalate prodrugs, is not suitable for prolonged or repeated courses owing to the risk of carnitine depletion.
Take with adequate fluids while seated or standing to reduce the risk of oesophageal irritation, and avoid in conditions causing oesophageal obstruction or stasis.

Monitoring

No routine laboratory monitoring is required for short treatment courses; reassess if symptoms do not resolve or recur.

Counselling the patient

Take the tablets with plenty of water and complete the full course.
Tell us if you have ever had an allergic reaction to penicillin.
Seek advice if your symptoms do not improve within a couple of days or if you develop a rash.

Evidence & guidelines

NICE guidance lists pivmecillinam as a first-line antibiotic option for uncomplicated lower urinary tract infection.

Reference: NICE NG109; UKHSA; 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

Centor/McIsaac Score (Pharyngitis) · Throat Infections

Pathways