Side Effects & Safety Melatonin is LIKELY SAFE for most adults when taken by mouth or injected into the body short-term, or when applied directly to the skin. Along with its needed effects, clonidine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention. SIDE EFFECTS. Most adverse effects have been mild and transient. The frequency estimates in the following table were derived from controlled studies in hypertensive patients in which adverse reactions were either volunteered by the patient (US studies) or elicited, eg, by checklist (foreign studies). Initially, 0.1 mg PO twice daily. Increase by 0.1 mg/day at weekly intervals until desired effect is achieved (usual range: 0.2 to 0.6 mg/day). Catapres (clonidine hydrochloride) is a centrally acting alpha-agonist hypotensive agent used to treat hypertension. Catapres is available as a generic named clonidine (tablets and patches). Clonidine comes in tablets of 0.1, 0.2, and 0.3 milligrams (mg), and your doctor will try to find the lowest dose of clonidine that works for you. Clonidine is used to treat hypertension (high blood pressure) and attention deficit hyperactivity disorder (ADHD). Learn about side effects, interactions and indications. Side Effects. Dizziness, lightheadedness, drowsiness, tiredness, stuffy nose, dry mouth, or constipation may occur. If any of these effects persist or worsen, tell your doctor or pharmacist promptly. Extended-release clonidine (Kapvay) is an oral drug used to treat ADHD. Learn how it works, its side effects, drug interactions, and more. However, it may cause some side effects. Some of the side effects of Clonidine 0.1 mg are as follows: 1) An individual taking it may face constipation.