**Core Concept**
Fluoxetine, a selective serotonin reuptake inhibitor (SSRI), is a commonly prescribed antidepressant medication. The most common side effects of fluoxetine are related to its mechanism of action, which involves increasing the levels of serotonin in the synaptic cleft.

**Why the Correct Answer is Right**
The most common side effect seen with fluoxetine therapy is gastrointestinal disturbance, particularly nausea and vomiting. This is due to the increased levels of serotonin in the gut, which can cause smooth muscle contraction and increased peristalsis. The mechanism involves the stimulation of 5-HT3 receptors in the enteric nervous system, leading to the activation of vagal afferent neurons and the subsequent vomiting response. Additionally, fluoxetine can also cause changes in appetite, weight gain or loss, and changes in bowel habits.

**Why Each Wrong Option is Incorrect**
**Option A:** Headache is a common side effect of many medications, including SSRIs, but it is not the most common side effect seen with fluoxetine therapy.
**Option B:** Dizziness and lightheadedness can occur with fluoxetine, but they are not the most common side effects.
**Option C:** Fluoxetine can cause sexual side effects, such as decreased libido and erectile dysfunction, but these are not the most common side effects.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the most common side effects of fluoxetine can be mitigated by starting with a low dose and gradually increasing it, as well as by advising patients to take the medication with food to reduce gastrointestinal upset.

**Correct Answer: A. Nausea and vomiting**