Mephentermine, sold under the brand name Wyamine among others, is a sympathomimetic medication which was previously used in the treatment of low blood pressure but is mostly no longer marketed. It is used by injection into a vein or muscle, by inhalation, and by mouth.

Side effects of mephentermine include dry mouth, sedation, reflex bradycardia, arrhythmias, and hypertension. Mephentermine induces the release of norepinephrine and dopamine and is described as an indirectly acting sympathomimetic and psychostimulant. Its sympathomimetic effects are mediated by indirect activation of α- and β-adrenergic receptors. Chemically, it is a substituted phenethylamine and amphetamine and is closely related to phentermine and methamphetamine.

Mephentermine was first described and introduced for medical use by 1952. It was discontinued in the United States between 2000 and 2004. The medication appears to remain available only in India. Misuse of mephentermine for recreational and performance-enhancing purposes has been increasingly encountered in modern times, especially in India.

For maintenance of blood pressure in hypotensive states, the dose for adults is 30 to 45 mg as a single dose, repeated as necessary or followed by intravenous infusion of 0.1% mephentermine in 5% dextrose, with the rate and duration of administration depending on the patient's response.^{[citation needed]}

For hypotension secondary to spinal anesthesia in obstetric patients, the dose for adults is 15 mg as a single dose, repeated if needed. The maximum dose 30 mg.^{[citation needed]}

Mephentermine has also been used as a decongestant.

Mephentermine is available in the form of 15 and 30 mg/mL solutions for intravenous infusion or intramuscular injection and in the form of 10 mg oral tablets. It has also been available in the form of inhalers.

Low blood pressure caused by phenothiazines, hypertension, and pheochromocytoma.^{[citation needed]}