RecruitingPhase IIInterventional

A clinical trial of clomifene citrate in men with infertility

Many couples struggle to conceive due to male fertility problems. Sometimes this is linked to lower hormone levels, while other times doctors cannot find a clear cause. Clomifene citrate is a well-known tablet used to treat female infertility, but doctors believe it might also boost sperm production in men by encouraging the body to produce natural reproductive hormones. The CONCRETE study is testing whether taking a low daily dose of clomifene citrate for nine months can improve sperm count and pregnancy chances compared to a dummy pill (placebo). Around 160 men across UK NHS hospitals will take part. Participants will have regular health checks, blood tests, and semen analyses over 18 months. This research aims to provide clear evidence on whether clomifene is a safe, effective NHS treatment option for men experiencing fertility challenges.

At a glance

Status

Recruiting

Phase

Phase II

Sponsor

University College London Comprehensive Clinical Trials Unit

Enrolment target

160

Start

01 Aug 2026

Estimated completion

31 Jul 2027

Male infertility

What is this study about?

Infertility affects many couples across the UK, and in around half of all cases, male fertility challenges play an important role. In some men, the brain and the testicles do not communicate properly, which can lower testosterone levels and reduce sperm count. In many other men, doctors cannot find an obvious physical cause for low sperm numbers.

At present, there are very few licensed medical treatments for male infertility on the NHS. Clomifene citrate is a tablet that has been used safely for decades to help women release eggs. It works by prompting the brain to release natural signals that stimulate the reproductive system. In men, these same signals encourage the testicles to produce more testosterone and generate healthier sperm.

Because clomifene is not yet officially approved for male infertility in the UK, robust scientific evidence is needed to prove how well it works and how safe it is. The CONCRETE study compares clomifene against an inactive dummy pill (placebo) to measure its true impact on sperm quality and natural conception rates.

If the study shows positive results, it could pave the way for a simple, accessible tablet treatment on the NHS. This could give couples a straightforward option to try before considering invasive and stressful procedures such as IVF.

Key takeaways

Tests whether a daily tablet (clomifene citrate) can improve sperm count and natural pregnancy rates.
Open to men aged 18 to 44 who have been trying for a baby for at least 12 months.
Involves taking one tablet daily for 9 months, followed by check-ups until month 18.
Includes hospital visits for semen analysis, blood tests, and health questionnaires.
Participation is voluntary and will not impact your standard NHS fertility support.

Who may be eligible?

This study is open to men aged 18 to 44 who have been actively trying for a baby with their partner for at least a year without success. To be eligible, your medical tests must show a reduced sperm count (16 million sperm per millilitre or fewer) or lower testosterone levels caused by hormone signalling problems.

You cannot take part if your fertility challenges are caused by physical blockages, chromosomal or genetic conditions, or previous medical treatments like chemotherapy or radiotherapy. Men who have had undescended testicles, certain serious infections of the testicles (such as mumps orchitis), or untreated swollen veins in the scrotum (varicoceles) are also unable to join.

In addition, you will not be able to participate if you have ever used testosterone treatments, anabolic steroids, or other hormone therapies, as these can interfere with your body's natural sperm production for long periods.

Could this study suit you?

Answer these quick questions to see if you may be eligible. This is a guide only — the research team makes the final call.

I am a man aged between 18 and 44 years old.
My partner and I have been trying to conceive for at least 12 months.
I have been told I have a low sperm count or lower hormone levels.
I have never taken testosterone therapy, clomifene, or anabolic steroids.
I do not have known physical blockages or genetic causes of infertility.

Answer every question to see your result.

What does participation involve?

If you decide to join the study, you will be randomly assigned to take either one clomifene citrate tablet (25 mg) or one identical dummy tablet (placebo) each day for nine months. Neither you nor your hospital research team will know which tablet you have received, ensuring the study remains fair.

You will attend hospital clinic visits at 3, 6, and 9 months. At these appointments, you will provide semen samples, have routine blood tests, and fill out short surveys about your general health and sexual wellbeing. After completing nine months of tablets, the study team will monitor your progress for another nine months, finishing with a final health check at 18 months to check on your wellbeing and record any pregnancies.

Potential risks and benefits

Taking part could potentially increase your sperm count and improve your chances of starting a family, though these benefits cannot be guaranteed. You will also receive thorough, regular health check-ups from fertility specialists. Clomifene is generally safe, but some men may experience mild side effects such as mood changes, headaches, hot flushes, or temporary changes in vision. Your health will be monitored closely throughout the study. Joining is entirely your choice, and you are free to leave at any time without your regular NHS care being affected.

Locations (7)

University College London Hospital
City only
London, England
Epsom and St Helier University Hospitals NHS Trust
City only
Carshalton, England
Royal Victoria Infirmary
Approximate
Newcastle upon Tyne, England
Southmead Hospital
Approximate
Bristol, England
St James' University Hospital
Approximate
Leeds, England
St Marys Hospital
Unverified
Manchester, England
Guy's and St Thomas' NHS Foundation Trust
Unverified
London, England

Common questions

What is a placebo and why is it used?

A placebo is a dummy pill that looks identical to the real medication but contains no active drug. It helps researchers find out whether any improvements are truly caused by the medicine.

Will I find out which tablet I was taking?

Neither you nor your doctors will know during the study. Once the entire trial is finished and all the results are analysed, you can find out which treatment you received.

Can my partner and I keep trying for a baby during the study?

Yes. You are encouraged to keep trying naturally, as the study aims to see whether the tablet helps couples achieve a pregnancy.

Does it cost anything to take part?

No. All study appointments, laboratory tests, and trial medications are completely free through the NHS.

What should I do if I notice any side effects?

You should contact your study doctor or nurse straight away. They can examine you and decide whether to alter your treatment or pause the tablets.

How to find out more

Rumana Jalil

cctu.concrete@ucl.ac.uk +44 20 3108 6584 Official trial record

Always speak to your GP or specialist before deciding to take part in a study.