Sleeplessness is the next most common side effect, presenting either in the form of having a hard time falling asleep or of awakening very early in the morning and not being able to get back to sleep. This occurs primarily in the first week of Phentermine use subsiding to the level of a nuisance; however, for some patients it can linger for a few weeks, lessening gradually over this time. Constipation also occurs fairly frequently and lingers for some, but except in rare cases, it can usually be managed by being aggressive in terms of water and fiber intake. Mild headaches, restlessness, anxiety, irritability, and intermittent heart racing are not uncommon in the first few days of a new user starting Phentermine, but usually, diminish quickly throughout the first week.