Pivmecillinam (Selexid)
Is It Right for You?
A complete guide to Pivmecillinam (Selexid) — what it treats, how it works, dosages, side effects, and when a clinician may prescribe it following an online consultation.
What Is Pivmecillinam (Selexid)?
Pivmecillinam is an orally administered antibiotic belonging to the penicillin class, specifically an amidinopenicillin. It is the pivaloyloxymethyl ester prodrug of mecillinam (amdinocillin) — meaning it is pharmacologically inactive when taken by mouth but is rapidly hydrolysed in the gastrointestinal mucosa to the active antibiotic mecillinam, which then achieves therapeutic concentrations in urine. This prodrug design overcomes the poor oral bioavailability of mecillinam itself.
Pivmecillinam is marketed under the brand name Selexid and is available as 200 mg tablets. It has a narrow but clinically important spectrum of activity, directed predominantly against aerobic Gram-negative bacteria — particularly the Enterobacteriaceae family — that cause the majority of urinary tract infections (UTIs). Crucially, mecillinam has a distinct mechanism of action compared with other penicillins, which confers a different resistance profile: many bacteria resistant to standard penicillins and some resistant to trimethoprim or nitrofurantoin remain susceptible to mecillinam.
Pivmecillinam is recommended by NICE and Public Health England as one of the first-line antibiotic options for uncomplicated lower urinary tract infections (cystitis) in women, alongside nitrofurantoin, trimethoprim, and fosfomycin. It is particularly valued in the context of antimicrobial stewardship — the imperative to preserve the effectiveness of existing antibiotics — as its narrow spectrum minimises disruption to the normal bowel microbiome.
What Conditions Does Pivmecillinam Treat?
Pivmecillinam is prescribed for:
This is the primary and most common indication for pivmecillinam in UK primary care. Uncomplicated lower UTI (cystitis) presents with dysuria (painful urination), urinary frequency and urgency, suprapubic discomfort, and cloudy or malodorous urine, without features of upper urinary tract involvement (no fever, loin pain, rigors, or systemic illness). The causative organism in approximately 80% of community-acquired uncomplicated UTIs is Escherichia coli, which is reliably susceptible to mecillinam. Klebsiella pneumoniae and Staphylococcus saprophyticus are other common organisms that remain susceptible.
Pivmecillinam is particularly useful as an alternative when first-line options (nitrofurantoin, trimethoprim) are contraindicated, not tolerated, or when local antibiotic resistance patterns make them inappropriate.
Women with recurrent UTIs (defined as three or more infections per year or two within six months) may be managed with self-start antibiotic therapy using pivmecillinam, whereby a clinician provides a prescription in advance for the patient to initiate treatment at symptom onset, without waiting for a consultation.
UTIs during pregnancy require prompt treatment because of the significantly increased risk of upper urinary tract involvement (pyelonephritis), preterm labour, and low birth weight compared with non-pregnant women. Pivmecillinam is considered a relatively safe option in pregnancy when nitrofurantoin is unsuitable (particularly in the third trimester, when nitrofurantoin is avoided near term). The clinical decision should always weigh available options, gestational age, and organism sensitivity.
While UTIs in men are uncommon, when they occur they are technically considered complicated due to the potential involvement of the prostate or urinary tract abnormality. Pivmecillinam may be used under clinical guidance, though investigation of the underlying cause is important.
While Pivmecillinam is effective for managing UTIs, it should be used cautiously in patients with a history of penicillin allergy or renal impairment.
How Does Pivmecillinam Work?
Mecillinam's mechanism of action is unique among penicillins. Standard penicillins (amoxicillin, flucloxacillin) primarily bind to and inhibit penicillin-binding protein 1A and 1B (PBP-1A/1B) in the bacterial cell wall synthesis machinery, causing lysis of rapidly dividing bacteria. Mecillinam, by contrast, has a uniquely high affinity for PBP-2 — a different penicillin-binding protein involved in maintaining the characteristic rod shape of Gram-negative bacteria.
When mecillinam inhibits PBP-2, bacteria lose their normal elongated rod shape and transform into spherical forms (spheroplasts). These spherical cells are osmotically fragile and structurally unstable — they cannot maintain cell wall integrity and ultimately lyse (burst), killing the bacteria. This distinct mechanism means that mecillinam retains activity against many bacteria that have developed resistance to other beta-lactam antibiotics and does not share cross-resistance patterns with most standard penicillins.
Mecillinam concentrates in urine following oral pivmecillinam administration, achieving concentrations far exceeding the minimum inhibitory concentrations (MICs) of susceptible urinary pathogens — providing highly effective local bactericidal activity within the urinary tract.
Dosages & Administration
The correct dose depends on the type and severity of infection, age, weight, and kidney function. Always follow your clinician's instructions. The table below is for general reference only.
Administration Tips
Pivmecillinam tablets should be taken with at least half a glass of water (approximately 100–200 ml) and swallowed in the upright position. Swallowing without adequate water has been associated with oesophageal ulceration due to local mucosal exposure to the drug. Tablets may be taken with or without food, but taking with food reduces gastrointestinal side effects.
The full course should always be completed as prescribed, even if symptoms resolve before the course is finished, to ensure the infection is fully eradicated and to minimise the risk of resistance development
Side Effects of Pivmecillinam
Pivmecillinam is generally well tolerated, but some side effects may occur.
- Nausea
- Diarrhoea
- Abdominal discomfort
- Vomiting
- Severe allergic reaction (anaphylaxis) — swelling of face, lips, or throat, difficulty breathing
- Severe skin reactions — blistering, peeling skin (e.g. Stevens-Johnson syndrome, rare)
- Severe diarrhoea — possible Clostridioides difficile infection
- Blood disorders (rare) — unusual bruising, bleeding, or infections
Patients should be informed about the risk of allergic reactions and the importance of completing the full course of treatment.
Drug Interactions
Inform your clinician about all medications you are taking, as some drugs can interact with Pivmecillinam and affect its efficacy.
May decrease renal excretion of pivmecillinam, increasing plasma concentrations. This interaction is sometimes used therapeutically but requires caution.
Important Warnings
Always swallow pivmecillinam tablets with at least half a glass (100–200 ml) of water while sitting upright or standing. Inadequate water intake has caused oesophageal ulceration. Do not lie down immediately after taking the tablet.
Inform your clinician of any previous allergic reaction to penicillin or any other antibiotic before taking pivmecillinam. If you experience rash, swelling, difficulty breathing, or any sign of an allergic reaction after taking pivmecillinam, stop immediately and seek medical help.
Always complete the full prescribed course of pivmecillinam, even if your UTI symptoms resolve before it is finished. Stopping early may leave some bacteria alive and contributing to antibiotic resistance.
Speak to a Clinician About Treatment
Getting advice no longer means sitting in a waiting room. Through The GP Service, you can consult with a licensed clinician in minutes — from home, on your lunch break, or wherever works for you. If treatment is clinically appropriate, your clinician can issue a prescription during the consultation. A consultation does not guarantee a prescription.
The treatment you need, when you need it.
Pivmecillinam FAQs
Trimethoprim was historically a standard first-line UTI antibiotic in the UK, but resistance rates among E. coli (the most common UTI pathogen) have risen substantially in many areas — in some regions exceeding 30%. NICE guidance now recommends checking local resistance data before prescribing trimethoprim and considers pivmecillinam, nitrofurantoin, and fosfomycin as preferred first-line alternatives. Pivmecillinam has lower resistance rates against common UTI pathogens and its narrow spectrum is beneficial from an antimicrobial stewardship perspective.
No. Pivmecillinam is appropriate for uncomplicated lower UTI (cystitis) only. Upper UTI (pyelonephritis) causes systemic symptoms including fever, rigors, loin pain, and general illness, and requires a different class of antibiotic (typically co-amoxiclav, a quinolone, or IV cephalosporins in hospital) and a longer treatment course. If you develop these symptoms, seek medical review promptly rather than self-treating with pivmecillinam.
Most patients notice improvement in urinary symptoms (burning, urgency, frequency) within 24 to 48 hours of starting treatment. However, the full course must be completed regardless of symptom resolution to ensure eradication of the infection.
Yes. Current UK guidance does not recommend additional contraceptive precautions when taking pivmecillinam alongside combined or progestogen-only oral contraceptive pills. The older concern that antibiotics reduce pill efficacy by disrupting gut bacteria that reactivate oestrogen has not been supported by clinical evidence for most antibiotics, including pivmecillinam.
Pivmecillinam can be used for lower UTI in men in selected circumstances under clinical guidance. However, UTIs in men are less common than in women and may indicate an underlying cause (prostatic enlargement, structural abnormality). Men with a UTI should always be assessed by a clinician rather than self-treating, and may require investigation such as urine culture, PSA measurement, or urological assessment.