Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link



Sheldon R. Morris

, MD, MPH, University of California San Diego

Reviewed/Revised Feb 2023

Trichomoniasis is a sexually transmitted infection of the vagina or urethra that is caused by the protozoa Trichomonas vaginalis and that causes vaginal irritation and discharge and sometimes urinary symptoms.

  • Women may have a greenish yellow, frothy, fishy-smelling vaginal discharge with irritation and soreness in the genital area.

  • Men usually have no symptoms, but a few have a frothy discharge from the penis and mild pain or discomfort during urination.

  • Examination of a sample of the discharge under a microscope usually enables doctors to identify trichomoniasis.

  • Infected people and their sex partners are treated with antibiotics.

  • Using condoms during genital sex can help prevent passing trichomoniasis and other sexually transmitted infections (STIs) from one person to another.

Trichomonas vaginalis commonly causes an STI of the vagina in women and an STI of the urinary tract in men and women. Women are much more likely to develop symptoms. About 20% of women develop trichomoniasis of the vagina (trichomonas vaginitis Trichomonal Vaginitis Trichomonal vaginitis is a vaginal infection caused by the protozoa Trichomonas vaginalis. Trichomonal vaginitis is usually sexually transmitted. It can cause a green or yellow discharge... read more ) during their reproductive years.

Symptoms of Trichomoniasis

In women, the infection usually starts with a greenish yellow, frothy, fishy-smelling discharge from the vagina. In some women, the discharge is slight. The genital area may be irritated and sore, and sexual intercourse may be painful. In severe cases, the genital area and surrounding skin may be inflamed, and the tissues around the vagina's opening (labia) may be swollen. Urination may be painful or frequent, as occurs in a bladder infection. Urinary and vaginal symptoms may occur alone or together.

Most men with trichomoniasis of the urethra (the tube carries urine from the bladder out of the body) have no or only mild symptoms, but they can still infect their sex partners. Some men have a frothy discharge from the penis, pain during urination, and an urge to urinate frequently.

Did You Know...

  • Most men with trichomoniasis have no symptoms, but they can still infect their sex partners.

Diagnosis of Trichomoniasis

  • Examination and sometimes culture of a sample of the discharge or urine

Doctors suspect trichomoniasis in the following people:

The organism is much more difficult to detect in men than in women.

In women, the diagnosis can usually be made quickly by examining a sample of the vaginal discharge with a microscope and identifying the organism. If results are unclear, the sample is cultured for several days.

In men, a sample of the discharge from the end of the penis (obtained in the morning, before urination) may be examined under a microscope and sent to the laboratory for culture. Alternatively, doctors sometimes do tests to identify the organism’s unique genetic material (DNA or RNA) using techniques that increase the amount of the bacteria's genetic material. These tests are called nucleic acid amplification tests (NAATS). These techniques make the organisms easier to detect.

Occasionally, microscopic examination of the urine detects Trichomonas, but identification is more likely if a urine culture is done.

Treatment of Trichomoniasis

  • The antibiotic metronidazole or tinidazole

  • Simultaneous treatment of sex partners

Women are usually treated with a seven-day course of antibiotics, either metronidazole or tinidazole. Men may be treated with a single dose. Their sex partners must be treated simultaneously or women may be reinfected. Sometimes, if a woman's partner is unlikely to return for follow-up, the woman is given medications or a prescription to deliver to her sex partner (called expedited partner therapy Sex partners Sex partners ).

Metronidazole may cause a metallic taste in the mouth, nausea, or a decrease in the number of white blood cells. Women who take the medication may be more susceptible to vaginal yeast infections Vaginal Yeast Infection (Candidiasis) A vaginal yeast infection (also called candidiasis) is caused by an infectious organism called Candida, usually Candida albicans. A vaginal yeast infection may cause intense itching... read more (vaginal candidiasis).

Infected people should abstain from sexual intercourse until the infection is cured, or they can infect their partners.

Prevention of Trichomoniasis

The following general measures can help prevent trichomoniasis (and other STIs):

Drugs Mentioned In This Article

Generic Name Select Brand Names
Flagyl, Flagyl ER, Flagyl RTU, LIKMEZ, MetroCream, MetroGel, MetroGel Vaginal, MetroLotion, Noritate, NUVESSA, Nydamax, Rosadan, Rozex, Vandazole, Vitazol
quiz link

Test your knowledge

Take a Quiz!