Brown brown is discussed in multiple episodes of the anime Shin Chan (dubbed by Funimation) throughout Season 3. Mitzi's sister Bitzi is addicted to the substance, spending $5000 on it in between episodes 63 and 64.
According to "The Lowdown on Brown-Brown" by Brendan I. Koerner, brown-brown is most likely an urban myth. Reasons being that cocaine would be difficult to get during armed conflicts, especially in the African Continent. Brown pills that were referred to as cocaine were most likely amphetamine. The first actual report of brown-brown that uses the name brown-brown, is by a Norwegian NGO in 2005 and it states the term refers to heroin.