Final answer:
The gastrointestinal adverse reaction to sedatives from the given options is constipation, which is known to be a side effect associated with many sedative drugs.
Step-by-step explanation:
The adverse reaction to sedatives that is a gastrointestinal reaction is constipation. Sedatives, along with their primary effects of inducing sleep or relaxation, can have several side effects. When examining the given options:
- Drowsiness is expected rather than adverse.
- Constipation is indeed a gastrointestinal side effect associated with many sedatives.
- Headache is a general side effect but not specifically gastrointestinal.
- Dizziness is also a general and expected side effect of sedatives.
Therefore, the answer is option 2) Constipation.