Phentermine cannot be purchased anywhere in the United Kingdom without a prescription. It is listed as a Schedule 3 controlled drug under the Misuse of Drugs Regulations 2001, according to Patient.co.uk.
Phentermine is an anorectic medication that acts by decreasing appetite. It is prescribed for weight loss along with exercise and a low-calorie diet. Phentermine is generally used over a short period of three to six weeks because it can be habit forming. According to MedlinePlus, side effects can include dry mouth, diarrhea, constipation and vomiting. Serious side effects include increased blood pressure, heart palpitations, restlessness, dizziness, insomnia and chest pain. Serious or unusual side effects require the advice of a doctor.