The bottom line is that phentermine works in just three to four hours. However, it may take a couple of weeks before you start to see the results of weight loss. Phentermine is absorbed through the gastrointestinal tract after being taken orally. After ingestion, it reaches peak concentrations in three to four and a half hours.
The half-life of the drug is around 25 hours, meaning it takes that amount of time for half the dose to be metabolized by the liver or excreted unchanged through urine. Phentermine starts to take effect very quickly after taking the initial dose. It is absorbed through the gastrointestinal tract and reaches its maximum blood concentrations after three to four and a half hours. You should start to feel the effects within four hours.