Complex sentence: Although the precise mechanism of action by which amphetamines improve the symptoms of ADHD remains unknown, it is thought that amphetamines decrease ADHD symptoms due to their effects on the central nervous system.
Split into simpler sentences: Although the precise mechanism of action by which amphetamines improve the symptoms of ADHD remains unknown. However, it is thought that amphetamines decrease ADHD symptoms due to their effects on the central nervous system.