Although levodopa can cross our BBB, but it will be decarboxylated to dopamine peripherally before reaching our CNS - only small amount of Levodopa will be left and available to pass our BBB.

So, instead of giving levodopa in high dose (which can cause lots of side effects), we give levodopa with peripheral decarboxylase inhibitors (e.g. carbidopa, benserazide) which can inhibit decarboxylation of levodopa in GIT and peripheral tissues. (By combining levodopa and peripheral decarobxylase inhibitors, we will have less side effects too)