References UK clinical guidance from the NHS, the British HIV Association (BHIVA), and the British Association for Sexual Health and HIV (BASHH). Last updated: April 2026.
Semen is the fluid released from the penis during ejaculation. It contains sperm cells plus fluids from the prostate gland, seminal vesicles and other glands. Understanding what semen contains and how it relates to sexual health helps with both fertility and STI prevention.
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What Semen Contains
| Component | Purpose |
|---|---|
| Sperm cells | Fertilisation of egg |
| Fructose | Energy for sperm |
| Enzymes | Help sperm reach the egg |
| Zinc, calcium | Support sperm function |
| Mucus | Protective and lubricating |
| Alkaline fluid | Neutralises vaginal acidity |
Semen and STI Transmission
Several STIs can be present in semen:
- HIV
- Chlamydia
- Gonorrhoea
- Hepatitis B
- Hepatitis C (less commonly)
Condoms prevent semen exchange and reduce STI transmission. U=U means that HIV-positive people with undetectable viral loads don’t transmit through semen.
Normal Semen
- Usually white or grey, sometimes yellowish
- Volume varies (2-5ml typical)
- Consistency varies — gel-like to watery
- Mild smell
When to Seek Help
- Blood in semen (haematospermia)
- Persistent changes in colour or smell
- Pain during ejaculation
- Fertility concerns
Read more: Hepatitis C and semen | STIs and infertility | Do condoms prevent STIs?
Frequently Asked Questions
Can STIs be in semen without symptoms?
Yes — many STIs are carried in semen silently.
Is blood in semen serious?
Usually not, but persistent cases should be checked.
Does semen volume affect fertility?
Volume matters less than sperm count and quality.
Related reading
- A to Z of Sex: Sexual Health Terms Explained
- Alcohol and HIV: What You Need to Know
- Anxiety and HIV: Coping and Getting Support
- Are All STIs Curable?
- Blood Problems and HIV: Anaemia, Low Platelets and More
Sources
- NHS — Sexual health conditions
- BASHH — UK clinical guidelines for STI management
- British HIV Association (BHIVA) — HIV treatment guidelines
- Terrence Higgins Trust — sexual health information
- UK Health Security Agency — STI surveillance data
Steve Page is a recognised expert on Sexually Transmitted Diseases (STDs) and STD treatments, having published numerous articles in peer-reviewed journals and presented his research at conferences around the world. He has an in-depth understanding of the latest medical research on STDs, and is an advocate for the development of new treatments and protocols to improve the health of those affected. In addition to his research, he has dedicated his career to understanding the causes and symptoms of STDs, as well as how to best treat those impacted.
