Phenazopyridine

Phenazopyridine

Clinical data
License data	US FDA: Phenazopyridine
Pregnancy category	B
Routes of administration	oral
ATC code	G04BX06 (WHO)
Legal status
Legal status	US: OTC OTC(Canada)
Identifiers
IUPAC name 3-phenyldiazenylpyridine-2,6-diamine
CAS Number	94-78-0
PubChem CID	4756
KEGG	D08346 Y
CompTox Dashboard (EPA)	DTXSID1023445
ECHA InfoCard	100.002.149
Chemical and physical data
Formula	C11H11N5
Molar mass	213.239 g/mol g·mol−1
(verify)

Phenazopyridine is a chemical which, when excreted into the urine, has a local analgesic effect. It is often used to alleviate the pain, irritation, discomfort, or urgency caused by urinary tract infections, surgery, or injury to the urinary tract. Phenazopyridine was discovered by Bernhard Joos, the founder of Cilag.

Medical uses

Phenazopyridine is prescribed for its local analgesic effects on the urinary tract. It is typically used in conjunction with an antibiotic when treating a urinary tract infection. Phenazopyridine is not an antibiotic, but used in conjunction with an antibiotic can speed the early period of recovery from such an infection. In this combination, phenazopyridine is taken for only a short time, typically two days, while the antibiotic is continued for longer. After two days, there is little evidence of any benefit from continued administration of phenazopyridine versus administration of an antibiotic only.

Phenazopyridine is also prescribed for other cases to relieve irritation or discomfort during urination. For example, it is often prescribed after the use of a catheter or after penile surgery which results in the irritation of the lining of the urinary tract.

Pharmacokinetics

The drug is administered as a tablet, in either 100 mg or 200 mg doses of phenazopyridine hydrochloride. The tablets have a light red, dark red or dark violet color, and are taken with food.

The full pharmacokinetic properties of phenazopyridine have not been determined. It has mostly been studied in animal models, but they may not be very representative of humans. Rat models have shown its half-life to be 7.35 hours, and that it is excreted 40% hepatically.

Its mode of action is not well known, and only basic information on its interaction with the body is available. It is known that the chemical has a direct topical analgesic effect on the mucosa lining of the urinary tract. It is rapidly excreted by the kidneys directly into the urine. Hydroxylation is the major form of metabolism in humans, and the azo bond is usually not cleaved. On the order of 65% of an oral dose will be excreted directly into the urine chemically unchanged.

Side effects

Phenazopyridine frequently causes a distinct color change in the urine, typically to a dark orange to reddish color. This effect is common and harmless, and indeed a key indicator of the presence of the drug in the body. Users of phenazopyridine are warned not to wear contact lenses, as phenazopyridine has been known to permanently discolor contact lenses and fabrics.

Phenazopyridine can also cause headaches, upset stomach (especially when not taken with food), or dizziness. Less frequently it can cause a pigment change in the skin or eyes, to a noticeable yellowish color. This is due to a depressed excretion via the kidneys causing a build up of the drug in the skin, and normally indicates a need to discontinue usage. Other such side effects include fever, confusion, shortness of breath, skin rash, and swelling of the face, fingers, feet, or legs. Long-term use may cause yellowing of nails.

Phenazopyridine should be avoided by people with glucose-6-phosphate dehydrogenase deficiency, because it can cause hemolysis (destruction of red blood cells) due to oxidative stress. It has been reported to cause methemoglobinemia after overdose and even normal doses. In at least one case the patient had pre-existing low levels of methemoglobin reductase, which likely predisposed her to the condition. It has also been reported to cause sulfhemoglobinemia.

Phenazopyridine is a type of azo dye. Other azo dyes, which were previously used in textiles, printing, and plastic manufacturing, have been implicated as carcinogenic agents that can cause bladder cancer. While phenazopyridine has never been shown to cause cancer in humans, evidence from animal models suggests that it is potentially carcinogenic.

Criticism

Because it does not kill the organisms causing a bladder infection, there is a risk that some patients will take phenazopyridine alone to relieve their symptoms instead of seeking a physician first and starting an appropriate antibiotic. The medicine serves to only mask the pain, and does not kill the infection, possibly allowing a simple bladder infection to become a more serious kidney infection if taken alone.

Since phenazopyridine discolors the urine, it interferes with the standard urine dipstick test done in a physician's office, especially the leukocyte esterase parameter. Physicians are then left with little option but to assume that the patient has a bladder infection, even though the severity and certainty of the diagnosis may be in question. This makes antibiotic choice difficult. Furthermore, once antibiotics are started, if the problem persists, the antibiotics themselves can adversely affect any follow-up urine tests that need to be done.

Other problems that may cause similar symptoms to urinary tract infections include: vaginitis, cervicitis, ovarian cysts, infection from sexually-transmitted diseases, simple urethral irritation (such as from a bubble bath or sexual intercourse), prolapsed bladder, menstrual problems, yeast infection, muscle strain, ectopic pregnancy, and other conditions. A normal urine test would be an indication that one of these conditions may be going on, but unfortunately, phenazopyridine invalidates the test, making it possible for the doctor to miss the diagnosis of one of these conditions.

Use of phenazopyridine is generally more appropriate once a diagnosis has been established by a medical provider and an antibiotic has been started to kill the infection.

Brand names

In addition to its generic form, phenazopyridine is distributed under the following brand names:

• Azo-Standard
• Baridium
• Nefrecil
• Phenazodine
• Prodium
• Pyridiate
• Pyridium
• Pyridium Plus
• Sedural
• Uricalm
• Uristat
• Uropyrine
• Urodine
• Urogesic

External links

• Information about phenazopyridine from the US National Library of Medicine