Which of these drugs is incorrectly paired with its medicinal use?

The pairing of amphetamines with convulsions is incorrect because amphetamines are stimulants that are primarily used to treat attention deficit hyperactivity disorder (ADHD) and narcolepsy, not convulsions. In fact, amphetamines can have stimulant effects that may increase neural activity and potentially lead to seizure activity in susceptible individuals, making it an inappropriate treatment for convulsions.

In contrast, marijuana is used to alleviate nausea associated with cancer treatments, narcotics like morphine are widely recognized for their effectiveness in pain relief and can induce sedation leading to insomnia, and codeine is correctly used for both cough suppression and providing pain relief. Each of those drug uses aligns well with medical practices and established pharmacological effects. Therefore, the pair that associates amphetamines with convulsions does not reflect common medical understanding or usage.