Several different things can cause convulsions in adults.
Some things that can cause convulsions in adults are seizures, reactions to medications, drug use such as overdosing on drugs including meth etc.
Convulsions can also be caused by a brain injury, such as trauma, stroke, brain infection, or a brain tumor.
If someone or yourself suffers from convulsions you need to see a doctor and or go to urgent care or the emergency room to get medical attention to find out exactly what is causing the convulsions.
People with epilepsy can have convulsions because of abnormal electrical activity in the brain.
So sometimes the convulsions can be caused by overdosing on medicines or drugs or it could be a reaction to other medications or even the brain injuries or the seizures.