This is a medical question of type : causes 
 What causes Urinary Retention ? 
 Urinary retention can result from
                
- obstruction of the urethra  - nerve problems  - medications  - weakened bladder muscles
                
Obstruction of the Urethra
                
Obstruction of the urethra causes urinary retention by blocking the normal urine flow out of the body. Conditions such as benign prostatic hyperplasiaalso called BPHurethral stricture, urinary tract stones, cystocele, rectocele, constipation, and certain tumors and cancers can cause an obstruction.
                
Benign prostatic hyperplasia. For men in their 50s and 60s, urinary retention is often caused by prostate enlargement due to benign prostatic hyperplasia. Benign prostatic hyperplasia is a medical condition in which the prostate gland is enlarged and not cancerous. The prostate is a walnut-shaped gland that is part of the male reproductive system. The gland surrounds the urethra at the neck of the bladder. The bladder neck is the area where the urethra joins the bladder. The prostate goes through two main periods of growth. The first occurs early in puberty, when the prostate doubles in size. The second phase of growth begins around age 25 and continues during most of a mans life. Benign prostatic hyperplasia often occurs with the second phase of growth.
                
As the prostate enlarges, the gland presses against and pinches the urethra. The bladder wall becomes thicker. Eventually, the bladder may weaken and lose the ability to empty completely, leaving some urine in the bladder.
                
More information is provided in the NIDDK health topic, Prostate Enlargement: Benign Prostatic Hyperplasia.
                
Urethral stricture. A urethral stricture is a narrowing or closure of the urethra. Causes of urethral stricture include inflammation and scar tissue from surgery, disease, recurring UTIs, or injury. In men, a urethral stricture may result from prostatitis, scarring after an injury to the penis or perineum, or surgery for benign prostatic hyperplasia and prostate cancer. Prostatitis is a frequently painful condition that involves inflammation of the prostate and sometimes the areas around the prostate. The perineum is the area between the anus and the sex organs. Since men have a longer urethra than women, urethral stricture is more common in men than women.1
                
More information is provided in the NIDDK health topic, Prostatitis: Inflammation of the Prostate.
                
Surgery to correct pelvic organ prolapse, such as cystocele and rectocele, and urinary incontinence can also cause urethral stricture. The urethral stricture often gets better a few weeks after surgery.
                
Urethral stricture and acute or chronic urinary retention may occur when the muscles surrounding the urethra do not relax. This condition happens mostly in women.
                
Urinary tract stones. Urinary tract stones develop from crystals that form in the urine and build up on the inner surfaces of the kidneys, ureters, or bladder. The stones formed or lodged in the bladder may block the opening to the urethra.
                
Cystocele. A cystocele is a bulging of the bladder into the vagina. A cystocele occurs when the muscles and supportive tissues between a womans bladder and vagina weaken and stretch, letting the bladder sag from its normal position and bulge into the vagina. The abnormal position of the bladder may cause it to press against and pinch the urethra.
                
More information is provided in the NIDDK health topic, Cystocele.
                
Rectocele. A rectocele is a bulging of the rectum into the vagina. A rectocele occurs when the muscles and supportive tissues between a womans rectum and vagina weaken and stretch, letting the rectum sag from its normal position and bulge into the vagina. The abnormal position of the rectum may cause it to press against and pinch the urethra.
                
Constipation. Constipation is a condition in which a person has fewer than three bowel movements a week or has bowel movements with stools that are hard, dry, and small, making them painful or difficult to pass. A person with constipation may feel bloated or have pain in the abdomen the area between the chest and hips. Some people with constipation often have to strain to have a bowel movement. Hard stools in the rectum may push against the bladder and urethra, causing the urethra to be pinched, especially if a rectocele is present.
                
More information is provided in the NIDDK health topic, Constipation.
                
Tumors and cancers. Tumors and cancerous tissues in the bladder or urethra can gradually expand and obstruct urine flow by pressing against and pinching the urethra or by blocking the bladder outlet. Tumors may be cancerous or noncancerous.
                
Nerve Problems
                
Urinary retention can result from problems with the nerves that control the bladder and sphincters. Many events or conditions can interfere with nerve signals between the brain and the bladder and sphincters. If the nerves are damaged, the brain may not get the signal that the bladder is full. Even when a person has a full bladder, the bladder muscles that squeeze urine out may not get the signal to push, or the sphincters may not get the signal to relax. People of all ages can have nerve problems that interfere with bladder function. Some of the most common causes of nerve problems include
                
- vaginal childbirth  - brain or spinal cord infections or injuries  - diabetes  - stroke  - multiple sclerosis  - pelvic injury or trauma  - heavy metal poisoning
                
In addition, some children are born with defects that affect the coordination of nerve signals among the bladder, spinal cord, and brain. Spina bifida and other birth defects that affect the spinal cord can lead to urinary retention in newborns.
                
More information is provided in the NIDDK health topics, Nerve Disease and Bladder Control and Urine Blockage in Newborns.
                
Many patients have urinary retention right after surgery. During surgery, anesthesia is often used to block pain signals in the nerves, and fluid is given intravenously to compensate for possible blood loss. The combination of anesthesia and intravenous (IV) fluid may result in a full bladder with impaired nerve function, causing urinary retention. Normal bladder nerve function usually returns once anesthesia wears off. The patient will then be able to empty the bladder completely.
                
Medications
                
Various classes of medications can cause urinary retention by interfering with nerve signals to the bladder and prostate. These medications include
                
- antihistamines to treat allergies      - cetirizine (Zyrtec)    - chlorpheniramine (Chlor-Trimeton)    - diphenhydramine (Benadryl)    - fexofenadine (Allegra)    - anticholinergics/antispasmodics to treat stomach cramps, muscle spasms, and urinary incontinence      - hyoscyamine (Levbid)    - oxybutynin (Ditropan)    - propantheline (Pro-Banthine)    - tolterodine (Detrol)    - tricyclic antidepressants to treat anxiety and depression      - amitriptyline (Elavil)    - doxepin (Adapin)    - imipramine (Tofranil)    - nortriptyline (Pamelor)
                
Other medications associated with urinary retention include
                
- decongestants      - ephedrine    - phenylephrine    - pseudoephedrine    - nifedipine (Procardia), a medication to treat high blood pressure and chest pain  - carbamazepine (Tegretol), a medication to control seizures in people with epilepsy  - cyclobenzaprine (Flexeril), a muscle relaxant medication  - diazepam (Valium), a medication used to relieve anxiety, muscle spasms, and seizures  - nonsteroidal anti-inflammatory drugs  - amphetamines  - opioid analgesics
                
Over-the-counter cold and allergy medications that contain decongestants, such as pseudoephedrine, and antihistamines, such as diphenhydramine, can increase symptoms of urinary retention in men with prostate enlargement.
                
Weakened Bladder Muscles
                
Aging is a common cause of weakened bladder muscles. Weakened bladder muscles may not contract strongly enough or long enough to empty the bladder completely, resulting in urinary retention.