The bottom line is that phentermine works in just three to 4 or 4 hours. However, it may take a couple of weeks before you start to notice that you're losing weight. Phentermine is absorbed through the gastrointestinal tract after ingestion by mouth. After ingestion, phentermine reaches peak concentrations in 3 to 4.4 hours.
The half-life of the drug is about 25 hours. The half-life is the time it takes for half the dose of the drug to be metabolized by the liver or excreted unchanged through the urine. Phentermine starts to work very quickly after taking an initial dose. Phentermine medications are absorbed through the gastrointestinal tract.
It will reach its maximum blood concentrations after three to 4.4 hours. You should start to feel the effects in about four hours.