Background: Garlic is this ancient medicinal plant which has a wide range of uses, including fighting against microbes, viruses, fungi, protozoa, and helminths. Allicin is the active ingredient in garlic and responsible for its antiparasitic effects. The current research was conducted to demonstrate the possible curative and preventative benefits of allicin on both adult and juvenile S. mansoni worms. Methods: For 96 hours, immature and adult S. mansoni worms were cultured with varying doses of allicin (25, 50, 100, and 200 µg/ml). Their motor activity and death rate were evaluated every 24 hours using light microscopy. The ultrastructural effects of allicin on worms, were assessed by scanning electron microscopy. The researchers used 40 male Swiss albino mice strain CD1, divided into four groups with 10 animals in each group. Allicin was administered to three groups at varied intervals after they were infected with S. mansoni cercariae. Parasitological studies, including worm burden, oogram pattern, and tissue egg load, were used to assess the efficacy of allicin on day 54 after infection. The results revealed a high significant difference in S. mansoni immature worms when incubated with allicin at doses of 50, 100, and 200 µg/ml. Also incubating adult worms of S. mansoni with allicin at concentrations of 100 µg/ml and 200 µg/ml were significant. In vivo, there was non-significant impact on worm load with high significant rise in dead egg percentage. Conclusion: These data suggest that allicin is effective against S. mansoni worms in vitro, but with little impact in vivo.