Sooo, I used to live in Tanzania and Ghana and traveled quite a bit around Africa. One thing you see a lot of is glue huffing, especially by street children, since it staves off hunger. It probably doesn't sound that bad but the effects are pretty terrible. Users kind of walk around in a daze with this vacant look, often have residue from the glue on their faces, and sometimes become violent. Brain and heart damage, as well as death can result.
Another really noxious drug I saw in East Africa was inhaling literal jet fuel. It's called "mafuta ndege" in Swahili which means plane oil in English. There is a black market around this and it's diverted from the airports. You can usually tell when people are on this stuff because they have no equilibrium at all. They are walking around just waving their limbs all over the place. It also causes hallucinations. As with the glue, it can cause blackouts and you often see users sprawled on the pavement in cities, just dead to the world. The effects of huffing are apparently pretty short lasting, I've heard. Motorcycle fuel is another solvent that gets used, and it's popular among street kids in some places like Addis Ababa, Ethiopia.
Yes,
@Zopiclone bandit is correct in speaking about Nyaope, which is a smokable mixture of heroin, Anti-retroviral drugs (ARVs) [HIV meds], and fillers like rat poison or "dagga" (weed). The idea that it could be cut with sand is very credible to me. It is a huge problem in Southern Africa in particular, where HIV rates are highest and use of ARVs is up too. The effects can include hallucinations and delusions.
Codeine is big in some countries. I definitely wouldn't say it's a horrific drug but it's interesting in terms of African drug culture. Users tend to pour it straight down their throats to avoid terrible tooth decay. I haven't personally seen this, only read about it online.
Finally, "Bluetoothing" will probably blow your mind as a creative and awful spin on drugging. The basic premise is that users inject themselves with heroin (nyaope) and then pull their own blood back up the syringe and inject it into a friend, as a way to share the high. As you can imagine, this is a fantastic way to also share all kinds of diseases. Bluetoothing is big in Southern Africa, where they seem to be adept at coming up with inventive ways to do drugs.
If I think of any more, I'll post them here. I'd love to know if anyone else knows of any I didn't post.