In the UK, the treatment route for a gut parasite depends on which parasite it is. Threadworms are treated through a pharmacy, usually without a prescription or a GP appointment. Confirmed protozoal infections such as giardia are treated by a GP with prescription medicines. And a few infections, including cryptosporidium in otherwise healthy people, are not treated with medicines at all. This guide walks through each route and what the evidence says about how well treatment works. It names the medicines a clinician might use, but which treatment is right for you, and whether you need one at all, is always a decision for the pharmacist, GP or specialist looking after you.
Why is there no single parasite treatment?
Gut parasites fall into two broad groups: single-celled protozoa such as giardia, cryptosporidium and cyclospora, and worms such as threadworms and tapeworms. The medicines that clear worms do nothing to protozoa, and the reverse is also true. Even within the protozoa, one drug does not cover all of them. The medicines used for giardia do not work against cyclospora, for example.
That is why treatment starts with knowing what you are dealing with. A GP will normally confirm the parasite with a stool sample before prescribing anything; our guide to how stool tests for parasites work explains what that involves. If you want a refresher on the organisms themselves, we cover the common gut parasites in the UK in a separate guide.
Why is the pharmacy the first stop for threadworms?
Threadworms are the exception to everything that follows. For most families, no GP appointment is needed. The NHS advice is to speak to a pharmacist, who can supply a medicine called mebendazole without a prescription. They are also extremely common. A 2025 review estimates that around 30% of children worldwide are affected at some point, so a case in your household says nothing about your hygiene.
Because the eggs spread so easily, the NHS advises treating everyone in the household over the age of two at the same time, even people with no symptoms. Medicine alone is not enough, though. The tablets kill the worms but not the eggs, which survive on hands, bedding and surfaces for up to two weeks, so treatment is paired with a hygiene routine: regular hand washing, morning showers, hot washing of bedding and nightwear, and short fingernails. A 2019 review for European clinicians found that when threadworms come back, it is usually reinfection rather than failed treatment, which is why the household approach and the hygiene measures matter as much as the medicine.
The pharmacy route is not for everyone. The NHS advises telling the pharmacist if the person needing treatment is under two, pregnant or breastfeeding, because the usual medicine may not be suitable and a GP may need to decide instead. Hygiene measures alone, kept up for six weeks, are the NHS alternative when medicine is not appropriate. Prescribers make these calls using the NICE Clinical Knowledge Summary on threadworm and the British National Formulary. For what parents typically notice and how the infection is confirmed, see our guide to threadworms in children.
How is giardia treated?
Giardia is different. Treatment is prescription-only. Once a stool test confirms the infection, a GP will usually prescribe a short course of antibiotics, most commonly metronidazole or tinidazole, the standard first-choice medicines. According to the NHS, symptoms usually stop within about a week of starting treatment, and the people you live with sometimes need to be tested and treated as well.
The evidence behind these medicines is substantial. A review pooling 30 randomised trials, 3,930 people in total, supports their effectiveness, although it noted that the quality of the included trials varied a lot. Two pieces of NHS advice are worth knowing before you start: drink plenty of fluids, and do not drink alcohol while you are taking the antibiotics, because alcohol can react with the main medicines used for giardiasis.
For how the infection spreads, how long it lasts and the long-term symptoms it can leave behind, see our guide to giardia.
What happens if the first treatment does not work?
Sometimes symptoms carry on after the first course, and this is more common than most people expect. A 2022 systematic review found that in some settings up to 45% of giardia infections are not cleared by first-line treatment, and that simply repeating the same drug rarely works. Combinations of medicines, or a switch to a different drug, do considerably better.
This is exactly why persistent symptoms should go back to the GP rather than being waited out, because established next steps exist. In that review, a medicine called mepacrine (also known as quinacrine) cleared 88.8% of hard-to-treat infections across the included reports, and a Swedish series of 87 patients recorded a 94% cure rate with it. In the UK, a 2026 study from the Hospital for Tropical Diseases in London reviewed 531 giardia cases seen between 2020 and 2024. Around a third did not respond to first-line treatment, and the centre's stepped approach, a combination of two medicines as second line and mepacrine as third line, was successful in nearly all cases and well tolerated.
None of this is a list of treatments to ask for by name. It is the reason to go back. Your GP can try a different approach or refer you to a specialist centre, and the evidence says those next steps usually work.
Why is cryptosporidium treated differently?
Cryptosporidium breaks the pattern. For otherwise healthy people there is no specific medicine to take. The infection normally clears on its own within about two weeks, and UKHSA guidance focuses on drinking plenty of fluids while it does. A 2024 review of 134 studies confirms that in healthy people the illness is self-limiting.
The picture changes if your immune system is weakened, for example by HIV, chemotherapy or the medicines taken after a transplant. The same review found the infection can become chronic and dangerous in this group, and a major Lancet Infectious Diseases review, co-authored by the head of the UK's Cryptosporidium Reference Unit, notes there is no reliably effective drug where immunity is low. This is specialist territory. Tell your GP about your medical history and any medicines you take, and never stop or change an existing medicine on your own. Our cryptosporidiosis guide covers the symptoms and how the infection spreads.
What about cyclospora and amoebiasis?
The medicines used for other gut parasites do not work against cyclospora, which is usually picked up through food or water abroad. According to a 2019 review, the effective treatment is a specific antibiotic combination, trimethoprim-sulfamethoxazole, known in the UK as co-trimoxazole. Diagnosis and prescription both run through your GP, and no pharmacy medicine covers it. Our Cyclospora guide explains the travel link in detail.
Amoebiasis, which is rarer in the UK and also usually linked to travel, is treated with prescription medicines too. The same rule applies. A confirmed diagnosis comes first, then treatment chosen by a clinician.
What helps recovery, whatever the parasite?
Whichever infection you had, the basics of recovery are the same. Diarrhoea loses fluid faster than most people replace it, so the priority is drinking plenty. Water and squash are fine, and a pharmacist may recommend oral rehydration sachets if you show signs of dehydration. If you are well hydrated, your pee should be light yellow or clear.
As your appetite returns, build back to normal eating gradually. Fibre-rich foods and fermented foods such as live yoghurt and kefir are reasonable additions while your gut settles, but the research on gut recovery after a parasite infection is limited, so treat them as support rather than a fix. No food or supplement treats a parasite infection.
If you are taking antibiotics for giardia, the NHS advice is not to drink alcohol until the course is finished.
When should you see a GP?
Some symptoms need a GP rather than patience, whatever you suspect the cause is:
- diarrhoea lasting more than seven days, which the NHS says needs an urgent GP appointment or a call to NHS 111
- blood in your poo, or bleeding from your bottom
- unexplained weight loss
- a change in your normal bowel habit lasting more than three weeks
- persistent vomiting, or signs of dehydration that are not improving
- symptoms that come back, or never fully settle, after treatment
Go sooner if your immune system is weakened, if the person who is unwell is under two, or if you are pregnant. Tell your GP about any recent travel abroad, because it changes what they test for. This guide is for information only. It does not replace advice from a clinician who knows your history, and no test or article can decide treatment for you.
Sources
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