Fisherman's Friend is a British brand of mint lozenges. While Lottie and Mildred were in 1960 (during The Case of the Forked Road), they bought some (for sixpence or less) in a belief that the version sold back then contained opium.
Unlike many things in these comics, Fisherman's Friend actually exists in the real world, but has apparently never had opium in it even when first manufactured in 1865. Coca-Cola did, however, have trace amounts of cocaine in it when it was first made, though this ended long before 1960 so the girls would have been disappointed there as well.