**Core Concept**
β2 agonists are a class of medications that stimulate β2 adrenergic receptors, leading to bronchodilation and smooth muscle relaxation. They are commonly used in the treatment of asthma and chronic obstructive pulmonary disease (COPD). However, their use is associated with several adverse effects.

**Why the Correct Answer is Right**
One of the notable adverse effects of β2 agonists is their potential to cause **tachyphylaxis**, a condition in which the body becomes less responsive to the medication over time. This is due to the downregulation of β2 adrenergic receptors, which reduces the effectiveness of the medication. Additionally, β2 agonists can cause **hypokalemia**, a condition characterized by low potassium levels in the blood, due to increased potassium excretion in the urine. This can lead to muscle weakness, fatigue, and other cardiac arrhythmias.

**Why Each Wrong Option is Incorrect**
* **Option A**: This option is incorrect because β2 agonists do not typically cause **hypertension** as an adverse effect. In fact, they can lower blood pressure by causing vasodilation.
* **Option B**: This option is incorrect because **diaphoresis** (excessive sweating) is not a direct adverse effect of β2 agonists. While they can cause tremors and anxiety, diaphoresis is not a common side effect.
* **Option C**: This option is incorrect because **diabetes mellitus** is not directly related to the use of β2 agonists. However, long-term use of these medications can mask the symptoms of hypoglycemia in diabetic patients.

**Clinical Pearl / High-Yield Fact**
It's essential to note that β2 agonists can cause **rebound bronchospasm** when used as a rescue medication for acute asthma attacks. This is because regular use of these medications can lead to tolerance and dependence, making it difficult to manage symptoms without them.

**Correct Answer: C. Hypokalemia**