A single dose of praziquantel 40 mg/kg is the first choice. The standard dose is one 600 mg tablet per 15 kg. It is better if all the drugs can be taken at one administration, but sometimes it has to be divided into 2 or 3 at intervals of 4 to 6 hours. It allows a one-dose therapy, but is expensive. Sometimes praziquantel needs to be administered repeatedly. The medication kills the adult worms rather than the eggs. Although tissue injury is principally caused by an immune reaction to the eggs, the product is clinically active. Praziquantel is undoubtedly the most effective medication with the fewest side-effects for the treatment of all species of schistosomes. The drug seems less effective in regions of West Africa where there has been a recent invasion of the parasite, possibly due to rapid re-infections. Diminished sensitivity or resistance to praziquantel are other possibilities. Apart from its activity against all human schistosomes including S. mekongi, S. intercalatum and S. mattheei, praziquantel is also very effective against most infections due to trematodes (except Fasciola) and cestodes, but not roundworms.

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There are no contra-indications and no major toxicity has been reported. It may be given in hepatic insufficiency. The drug, however, is not recommended during the first three months of pregnancy. Side-effects include abdominal pain, vomiting, diarrhoea and fever, probably caused by the reaction to the dead worms. In very severe infections, cases of rectorrhagia are observed. In patients with a haemorrhagic diathesis, this could be dangerous. There is a major risk if cysticercosis is present in the region. Serious complications can occur as a consequence of the death of cysticerci in the CNS following administration of the medication. In spinal and cerebral schistosomiasis, praziquantel is combined with corticosteroids. Which of the two medications contributes most to the clinical improvement is unclear.

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This is fairly expensive, is less effective and has many side-effects. In addition, it must be given for a week. It was developed about 1960 and was the most widely used medication up until 1985. Niridazole is particularly active against S. haematobium, less active against S. mansoni and practically inactive against S. japonicum. It is also effective against Entamoeba histolytica. It is administered orally at doses of 25 mg/kg/day for 7 days. There are numerous, reversible side-effects which are mild but potentially more serious, such as neuropsychiatric disorders, psychoses, epileptic insults and clinically silent ECG disorders. The drug is not indicated in cases of liver involvement, which restricts its use in the hepato-intestinal form. It is also relatively expensive and is now outdated and no longer commercially available.

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Metrifonate or trichlorfon is a very cheap and safe medication. It is an organophosphorus derivative belonging to the same group as some insecticides used in agriculture (anticholinesterase activity). No evidence could be found of a harmful cumulative effect of the drug. It is active only against urinary bilharziasis, including the occasional localisations of S. mansoni in the bladder wall, but strangely not against S. haematobium and S. mansoni when they are localised in the intestinal wall. Metrifonate is administered orally in single doses of 7.5 mg/kg. The preferred schedule is three doses with an interval of two weeks between each dose. A single administration of 10 mg/kg, however, appears sufficient for the mass suppressive treatment of urinary bilharziasis. At doses of more than 10 mg, too many side-effects are observed. The cure rate is higher when the treatment is repeated and increases from 21% with one dose to 65% with two doses and 84% with three doses, although this is still lower than the results with a single dose of praziquantel. On the other hand, some of the worms appear to be consistently resistant to metrifonate, irrespective of the dose. Metrifonate has a narrower spectrum than praziquantel. The drug is very effective against hookworms. It was, however, withdrawn from the market in 1998.

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Oxamniquine is only effective against S. mansoni. It is rapidly absorbed from the intestine and has a plasma half-life of approximately 2 hours. The drug is weakly toxic. Epileptic insults have been reported in patients with a history of cerebral lesions and drowsiness in children which can progress to subcomatose states. The urine can be coloured red. Like praziquantel, the drug can be administered to patients with hepatic lesions, even if advanced. Oxamniquine is at least as expensive as praziquantel. It is administered orally as a single dose (sometimes administration in 2 doses in children) at doses of 15 to 20 mg/kg in Latin America and 20 to 40 mg/kg in Africa. Some of the side-effects are undoubtedly the result of the death of adult worms, including in the lungs, with the formation of temporary infiltrates and with respiratory symptoms. It is used extensively in Latin America (Brazil), but is less indicated in Africa where S. mansoni is associated in many places with S. haematobium. It may be used if there is an inadequate response to praziquantel.

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This medication is registered for the treatment of schistosomiasis but rarely used. A recent discovery is that the medication induces glutathione S-transferase. This enzyme plays an important role in the detoxification of carcinogens such as benzene and aflatoxin. The product is now being studied as a potential chemoprotective agent (protection against cancer).

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Mirazid contains an extract (an oleo gum resin) from the stem of Commiphora molmol (syn. Commiphora myrrha), the myrrh tree. An alcohol extract of this plant followed by steam distillation produces resin and volatile oil. It is licenced and marketed for clinical use against Fasciola hepatica and schistosome infections in Egypt. Its exact place in therapy is not yet clear and its clinical activity is controversial at the moment. A dose of 10 mg/kg for three consecutive days has been proposed. Myrrh extracts also have a molluscicidal effect on the snail intermediate hosts, particularly on their eggs.