Does Phentermine keep you awake?

Yes, and it is on the label rather than only in reports: insomnia appears among the common adverse reactions in Phentermine's FDA prescribing information, published 2026-06-15. This label names its reactions without incidence rates, so it says the reaction occurs, not how often. The drugs on this site that list insomnia act on the brain's arousal chemistry: a stimulant, or an antidepressant that raises noradrenaline and dopamine. The wakefulness is the same pharmacology that suppresses the appetite, not a separate effect that happens to come along with it. That is why the time of day you take it matters more here than for any other symptom on this list, and why the label says when to take it rather than leaving it to preference. It does not follow the dose schedule, so it does not come and go with dose increases. What helps first: taking each dose at the time the label sets, and no later. Where the label says morning, an evening dose is the commonest single cause. Where it splits the dose between morning and evening, the evening dose is part of the treatment, and moving it is a question for the prescriber rather than a change to make alone. It stops being ordinary at a specific line: insomnia with a racing or irregular heartbeat, agitation, or any change in mood — low mood, unusual irritability, or thoughts of harming yourself. Mood change is not a wait-and-see symptom on this class of drug; where this page shows a boxed warning above, that warning is about exactly this.