Enema
An enema (plural enemata or enemas) is the procedure of introducing liquids into the rectum and colon via the anus.  Enemas can be carried out for medical reasons (as a treatment for constipation), as a remedy for encopresis, as part of alternative health therapies, and also for erotic purposes, particularly as part of BDSM activities. In earlier times, they were often known as clysters, and were probably used more frequently than at present.

# Medical usage
The main medical usages of enemas are:
- As a bowel stimulant, not unlike a laxative -- the main difference being that laxatives are commonly thought of as orally administered while enemas are administered directly into the rectum, and thereafter, into the colon.  When the enema injection into the rectum is complete, and after a set "holding time," the patient expels feces along with the enema in the toilet.
- Enemas may also be used to relieve constipation and fecal impaction, although in many professional health-care settings, their use has been largely replaced by oral laxatives and laxative suppositories.  In-home use of enemas for constipation and alternative health purposes is somewhat harder to measure.
- Bowel stimulating enemas usually consist of water, which works primarily as a mechanical stimulant, or they may be made up of water with baking soda (sodium bicarbonate) or water with a mild hand soap dissolved in it; sodium phosphate solution, which draws additional water from the bloodstream into the colon and increases the effectiveness of the enema, but which can often be rather irritating to the colon, causing intense cramping or "gripping"; or mineral oil, which functions as a lubricant and stool softener, but which often has the side effect of sporadic seepage from the patient's anus which can soil the patient's undergarments for up to 24 hours.  Other types of enema solutions are also used, including equal parts of milk and molasses heated together to slightly above normal body temperature. In the past, castile soap was a common additive in an  enema, but it has largely fallen out of use because of its irritating action in the rectum and because of the risk of chemical colitis as well as the ready availability of other enema preparations that are perhaps more effective than soap in stimulating a bowel movement. At the opposite end of the spectrum, an isotonic saline solution is least irritating to the rectum and colon, having a neutral osmotic pressure, and neither leaching electrolytes from the body, as can happen with plain water, nor drawing water into the colon, as with phosphates, nor adding anything to the bloodstream.  Thus, a salt water solution can be used when a longer period of retention is desired, such as to soften an impaction.
- Cleansing the lower bowel prior to a surgical procedure such as sigmoidoscopy  or colonoscopy.  Because of speed and supposed convenience, enemas used for this purpose are commonly the more costly, sodium phosphate variety -- often called a disposable enema. A more pleasant experience preparing for testing procedures can usually be obtained with gently-administered baking soda enemas; cleansing the lower bowel for colonoscopy and other bowel studies can be effectively achieved with water-based, or water with baking soda, enema administration.
- The administration of substances into the bloodstream.  This may be done in situations where it is undesirable or impossible to deliver a medication by mouth, such as antiemetics given to reduce nausea (though not many antiemetics are delivered by enema). Additionally, several anti-angiogenic agents, which work better without digestion, can be safely administered via a gentle enema. Medicines for cancer, for arthritis, and for age-related macular degeneration are often given via enema in order to avoid the normally-functioning digestive tract. Interestingly, some water-based enemas are also used as a relieving agent for Irritable Bowel Syndrome, using cayenne pepper to squelch irritation in the colon and rectal area. Finally, an enema may also be used for hydration purposes. See also route of administration.
- The topical administration of medications into the rectum, such as corticosteroids and mesalazine used in the treatment of inflammatory bowel disease.  Administration by enema avoids having the medication pass through the entire gastrointestinal tract, therefore simplifying the delivery of the medication to the affected area and limiting the amount that is absorbed into the bloodstream.
- General anesthetic agents for surgical purposes are sometimes administered by way of an enema. Occasionally, anesthetic agents are used rectally to reduce medically-induced vomiting during and after surgical procedures, in an attempt to avoid aspiration of stomach contents.
- A barium enema is used as a contrast substance in the radiological imaging of the bowel.  The enema may contain barium sulfate powder, or a water-soluble contrast agent. Barium enemas are sometimes the only practical way to "view" the colon in a relatively safe manner. Following barium enema administration, patients often find that flushing the remaining barium with additional water, baking soda, or saline enemas helps restore normal colon activity without complications of constipation from the administration of the barium sulfate.
In certain countries such as the United States, customary enema usage went well into the 20th century; it was thought a good idea to cleanse the bowel in case of fever; also, pregnant women were given enemas prior to labor, supposedly to reduce the risk of feces being passed during contractions. Under some controversial discussion, pre-delivery enemas were also given to women to speed delivery by inducing contractions. This latter usage has since been largely abandoned, because obstetricians now commonly give oxytocin to induce labor and because women generally found the procedure unpleasant.

# Home usage
Many self-given enemas used at home are the pre-packaged, disposable, sodium phosphate solutions in single-use bottles sold under a variety of brand names, or in generic formats. Costing up to a dollar per use, these units come with a pre-lubricated nozzle attached to the top of the container. Some enemas are administered using so-called disposable bags connected to disposable tubing (despite the names, such units can commonly be used for many months or years without significant deterioration).
Patients who want easier, more gently-accepted enemas often purchase Combination Enema Syringes which are commonly referred to as "closed top" syringes, and which can also be used as old-fashioned hot water bottles, so as to relieve aches and pains via gentle heat administrations to parts of the body. Cost for each enema can be as little as a few pennies for the baking soda added to ordinary tap water.
In medical or hospital environments, reusable enema equipment is now rare because of the expense of disinfecting a water-based solution. For a single-patient stay of short duration, an inexpensive disposable enema bag can be used for several days or weeks, using a simple rinse out procedure after each enema administration. The difficulty comes in from the longer time period (and expense) required of nursing aides to give a gentle, water-based enema to a patient, as compared to the very few minutes it takes the same nursing aide to give the more irritating, cold, pre-packaged sodium phosphate unit.
For home use, disposable enema bottle units are common, but reusable rubber or vinyl bags or enema bulbs may also be used.  In former times, enemas were infrequently administered using clyster syringes. If such commercially-available items are not at hand, ordinary water bottles are sometimes used.

# Colonic irrigation
Colonic irrigation or colon hydrotherapy is a large-volume enema which cleanses the whole colon. Colonic irrigation was in vogue for health and hygienic purposes at the beginning of the 20th century (see John Harvey Kellogg) and remains popular as an alternative health therapy in many parts of the world. Advocates believe that, when carried out by trained personnel using clean or sterilized equipment, colonic irrigation is a safe and valuable tool for eliminating toxins from the body and restoring normal muscular activity in the colon. However, there have been a few cases of intestinal perforation due to improperly done colonic irrigations.  The actual medical benefit of colonic irrigation is controversial but may have value for many common health conditions.

# Enema Usage in Rectal Drug Administration
In rectal administration of dissolved drugs or alcohol an enema might be used to help increase the rate of absorption by cleaning the colon of feces first .
Enemas have also been used for ritual rectal drug administration such as balche, alcohol, tobacco, peyote, and other hallucinogenic drugs and entheogens, most notably by the Mayans and also some other American Indian tribes. Some tribes continue the practice in the present day. 
People who wish to become intoxicated faster have also been known to use enemas as a method to instill alcohol into the bloodstream, absorbed through the membranes of the colon. However, great care must be taken as to the amount of alcohol used. Only a small amount is needed as the intestine absorbs the alcohol more quickly than the stomach. Deaths have resulted due to alcohol poisoning via enema. 
For more information on rectal drug administration, see route of administration.

# Recreational usage
The paraphilia directed towards enemas is known as klismaphilia, the enjoyment of enemas.  Enemas may be used as part of BDSM activities for either males or females, or as a regular sexual activity for an individual or between partners.  In many cities, enemas are available as a service from practitioners in the sex industry to cater to klismaphiliac desires.   Enemas can be pleasurable to either sex, and in males, enemas can can stimulate the prostate gland.  Unexpected erections are common in medical settings, even if the person would otherwise consider it an unpleasant procedure.
An enema may also be used prior to anal sex or anilingus in order to enhance the sensation of intercourse, or to remove feces prior to sex, possibly reducing bacterial transmission and risk of infection, or just to reduce the possibility of fecal material adhering to the genitals or sex toys used during the following activity.

# Precautions
Improper administration of an enema may cause electrolyte imbalance (with repeated enemas) or ruptures to the bowel or rectal tissues resulting in internal bleeding, however these occourances are rare in healthy, sober adults. Internal bleeding or rupture may leave the individual exposed to infections from intestional bacteria. Blood resulting from tears in the colon may not always be visible, but can distinguished if the feces is unusually dark or has red hue. If intestional rupture is suspected, medical assistance should be obtained immediately. 
The enema tube and solution may stimulate the vagus nerve, which triggers an arrhythmia such as bradycardia. Enemas should not be used if there is an undiagnosed abdominal pain since the peristalsis of the bowel can cause an inflamed appendix to rupture.