The question of whether the prescription weight-loss medication phentermine is similar to “speed” arises because both substances are powerful central nervous system (CNS) stimulants. Phentermine is primarily used for short-term management of obesity, prescribed alongside diet and exercise. In contrast, “speed” is a street term referring to illicit amphetamines or methamphetamines, which are potent stimulants abused for their euphoric effects. While they share a common chemical lineage and produce overlapping effects, significant distinctions exist in their molecular structure, mechanisms of action, and regulatory status.
Structural Similarity and Legal Status
Phentermine and amphetamines, including methamphetamine, are chemically related because they both belong to the phenethylamine class of compounds. This shared backbone is responsible for their similar stimulant properties. However, the molecules are not identical; phentermine has an additional methyl group on the alpha-carbon atom. This minor structural difference significantly impacts its overall pharmacological profile.
This slight molecular variation is the basis for phentermine’s lower potency and reduced potential for abuse compared to most amphetamines. In the United States, this difference is formally recognized by the Drug Enforcement Administration (DEA) through the Controlled Substances Act. Phentermine is classified as a Schedule IV controlled substance, indicating a relatively low potential for abuse and dependence.
Conversely, pharmaceutical amphetamine preparations, such as those used to treat Attention-Deficit/Hyperactivity Disorder (ADHD) and narcolepsy, are classified as Schedule II controlled substances. This Schedule II designation is reserved for drugs with a high potential for abuse that may lead to severe psychological or physical dependence. The illicit substances commonly called “speed” inherently carry this higher risk profile.
How They Affect the Central Nervous System
Both phentermine and amphetamines exert their primary effects by increasing the concentration of specific neurotransmitters in the brain. These substances promote the release and inhibit the reuptake of norepinephrine and dopamine, which are key chemicals regulating mood, alertness, and appetite. The resulting physiological effects include increased heart rate, elevated blood pressure, and a general sense of stimulation.
Phentermine is often considered an “atypical” amphetamine because its central nervous system (CNS) stimulation is less intense and less selective than that of typical amphetamines. Phentermine primarily works on norepinephrine, which is the main driver of its appetite-suppressing effect. While it also increases dopamine, it does so to a lesser extent than amphetamine, which reduces its potential for euphoria.
Amphetamines, particularly illicit forms of speed, cause a more robust and indiscriminate increase in dopamine levels, which is closely associated with the brain’s reward pathway. This stronger dopaminergic effect contributes to the intense feeling of euphoria, increased sociability, and heightened sense of power sought by recreational users. The difference in the intensity and selectivity of this neurotransmitter modulation determines their different regulatory statuses.
Comparing Therapeutic and Recreational Use
Phentermine is strictly a prescription medication with a defined therapeutic purpose. It is approved for use as a short-term adjunct therapy, typically for a few weeks, in managing exogenous obesity for patients with a high Body Mass Index (BMI). The medical use is always accompanied by professional oversight, behavioral modification, and caloric restriction. The intent is temporary appetite suppression to kickstart weight loss.
The intended use of “speed,” a term often used for illicit forms of amphetamines, revolves around recreational abuse. Users typically seek the drug for its potent psychoactive effects, including intense euphoria, sustained wakefulness, and performance enhancement. These uses are non-medical, unregulated, and frequently involve higher doses and riskier routes of administration, leading to a far greater risk of adverse outcomes.
Even when prescribed for conditions like ADHD, therapeutic amphetamines are used to improve focus and concentration, not for euphoria, and are administered under strict medical protocols. The stark contrast in administration context highlights the practical difference between phentermine and illicit speed.
Potential for Dependency and Adverse Effects
The potential for dependency is a serious consideration for any stimulant. While phentermine has a lower abuse potential than Schedule II amphetamines, it is still classified as a controlled substance because of its capacity to cause psychological and physical dependence. Physicians must cautiously prescribe the medication for limited durations to mitigate this risk.
Common adverse effects for both classes of stimulants include cardiovascular issues like elevated blood pressure and tachycardia (rapid heartbeat). Other psychological effects, such as anxiety, insomnia, and restlessness, are frequently reported with both substances. However, the severity and likelihood of life-threatening events, such as stroke or psychosis, are significantly greater with the high, unregulated doses typical of recreational amphetamine use.
Abruptly stopping the use of either drug after prolonged or high-dose consumption can lead to withdrawal symptoms. For illicit amphetamine users, this can manifest as severe depression, paranoia, and extreme fatigue. While phentermine withdrawal is typically milder, consisting mainly of increased hunger and fatigue, a gradual dosage reduction is necessary to avoid a pronounced crash.