Phentermine 37.5 mg is a prescription medication that is used as a weight loss medication. It works as an appetite suppressant that affects the central nervous system. While it is effective for weight loss, it is important to understand that this medication came with certain side effects. Some of the common side effects of phentermine 37.5 mg include dry mouth, constipation, headache, and insomnia. Many users reported feeling jittery, anxious or restless which can also lead to irritability.