These drugs affects your brain, lungs and heart.
If your brain fails first then you obviously get brain damage.
If your lungs fail first then the brain won't have enough oxygen and you also get a brain damage.
If your heart fails first then your brain won't get enough blood, so it won't get enough oxygen, so you get a brain damage.
The brain will always be the first organ to get a PERMANENT damage, because neurons are the most sensitive cells in the body, and the only ones who can't completely restore their functionality after being damaged.
If you worry about brain damage don't use drugs or toxins, try something that wont only harm your brain but also your whole body so you wont have a chance, like guns or explosives. TATP is easy to make but be careful not to hurt innocent people.