Anus and rectum

Structure

The anal canal is 3–4 cm long and guarded by two sphincters. The external sphincter is striated muscle under voluntary control. The internal sphincter is smooth muscle, involuntary, and does not relax on command — it relaxes in response to gradual pressure and time. This single fact explains most of what makes anal penetration comfortable or painful.

Above the canal sits the rectum, roughly 12–15 cm, which curves. It is not a straight tube: the rectosigmoid junction angles away, which is why depth requires angle changes and patience rather than force. The anal canal has dense somatic innervation via the inferior rectal branch of the pudendal nerve; the rectum above the dentate line has visceral innervation that registers stretch and pressure rather than fine touch.

The rectum does not self-lubricate. It produces a small amount of mucus, which is not sufficient for friction-free penetration.

Why lubricant is not optional

Rectal tissue is a single-cell-thick columnar epithelium in places, compared with the stratified squamous lining of the vagina. It tears more easily and heals in a bacteria-rich environment. Micro-tears are the main reason receptive anal intercourse carries the highest per-act HIV risk of any sexual activity — roughly 138 per 10,000 exposures for receptive partners in the CDC's pooled estimates, compared with about 8 per 10,000 for receptive vaginal intercourse.

Use generous lubricant, reapply often, and use more than feels necessary. Silicone lubricant lasts longest but degrades silicone toys; water-based needs reapplication. Avoid numbing lubricants — pain is the feedback loop that prevents injury, and anaesthetised tissue is injured tissue that has stopped reporting.

Safe practice

• Go slowly. Allow several minutes of external and shallow stimulation before any depth.
• The receptive partner controls pace and depth. This is not etiquette; it is the only reliable safety mechanism.
• Anything inserted must have a flared base. Rectal foreign body retrieval is a routine emergency presentation and is entirely preventable.
• Never move from anus to vagina or mouth without changing the condom, glove or washing. Gut bacteria in the vagina or urethra causes bacterial vaginosis and urinary tract infection.
• Trim nails, or use gloves; fingertip injuries to rectal mucosa are common and invisible.
• Stop for bright red bleeding, sharp pain, or pain that persists after stopping.

Douching: the evidence

Douching is not required. Faeces is stored in the sigmoid colon, not the rectum, between bowel movements. Observational studies (Carballo-Diéguez et al., 2008; Li et al., 2019) associate frequent rectal douching with mucosal damage and higher rates of HIV and other STIs. If douching, use plain lukewarm water or isotonic saline, low volume, low pressure, and not immediately before sex. Do not use tap water repeatedly in large volumes, soap, or commercial enemas containing sodium phosphate, which can cause electrolyte disturbance.

The prostate and the anterior wall

For people with a prostate, it sits 5–7 cm in on the anterior wall (toward the belly). Firm, broad pressure rather than jabbing is usually effective. For people without a prostate, the anterior rectal wall shares tissue planes with the internal clitoris and vagina and can be a source of sensation for the same reason.

Health

Haemorrhoids, fissures and skin tags are common and mostly not caused by anal sex. Persistent bleeding, a change in bowel habit lasting more than three weeks, unexplained weight loss, or a lump should be assessed and not attributed to sexual activity.

Anal HPV causes anal cancer; HPV vaccination protects against the responsible types, and anal cytology screening is offered in some settings to people living with HIV and other higher-risk groups. Incontinence is not a normal consequence of anal sex; if it occurs, ask for a pelvic floor assessment.

Sources

• CDC. "HIV risk behaviors: estimated per-act probability of acquiring HIV." Current edition.
• Patel P et al. "Estimating per-act HIV transmission risk: a systematic review." AIDS, 2014.
• Carballo-Diéguez A et al. "Rectal douching associated with receptive anal intercourse." AIDS and Behavior, 2008.
• Li P et al. "Association between rectal douching and HIV and other STIs among men who have sex with men: a meta-analysis." Sexually Transmitted Infections, 2019.
• BASHH. UK national guideline on the management of anogenital conditions, current edition.
• Stewart DB et al. "Clinical practice guideline for the management of anal fissures." Diseases of the Colon and Rectum, 2017.

Last reviewed: August 2026.