Giardiasis and bloody stool: what to know

Giardiasis is an intestinal infection caused by a parasite. It more often causes loose or watery diarrhoea, abdominal cramps, bloating, gas, nausea, and tiredness. Some people have few symptoms. Blood may come from a different cause, such as inflammation, another infection, or irritation near the anus, so its presence changes the advice even when giardiasis has been diagnosed.

A stool appearance can be difficult to judge, particularly after eating strongly coloured foods or taking certain medicines. Red blood, dark or tar-like stool, or blood mixed through diarrhoea should still be discussed promptly. Read more about [Giardiasis](/conditions/giardiasis/) and [Diarrhoea](/symptoms/diarrhoea/).

Arrange a [GP Consultation](/treatments/consultations-and-assessment/gp-consultation/) for advice about the bleeding and diarrhoea. Keep drinking fluids in small, frequent amounts, and note when the bleeding began, its colour, and whether it is mixed with stool. Wash hands carefully after using the toilet and before preparing food. Do not start leftover antibiotics or anti-diarrhoeal medicine without clinical advice; treatment depends on the cause.

Seek urgent medical help if bleeding is heavy or increasing, stool is black and tar-like, you feel faint or confused, you cannot keep fluids down, or you have severe abdominal pain, a swollen abdomen, or signs of dehydration such as very little urine. Children, older adults, pregnant people, and anyone with a weakened immune system should seek advice sooner.

Ask whether you need stool testing, whether the blood could have a cause other than giardiasis, and which symptoms should trigger urgent review. You can also ask whether [Antibiotic Treatment](/treatments/medical-management/antibiotic-treatment/) is appropriate after the assessment.