Which of the following is an effect of amphetamines?

Amphetamines are central nervous system stimulants that have several physiological effects. One of the well-documented effects of amphetamines is appetite suppression. This occurs because these drugs increase the release of neurotransmitters like norepinephrine and dopamine, which can enhance alertness and energy while concurrently diminishing feelings of hunger.

Increased sleep duration is not associated with amphetamines, as these substances typically lead to insomnia or reduced sleep time due to their stimulating effects. Similarly, amphetamines do not cause muscle relaxation; rather, they can lead to increased muscle tension and energy levels. Finally, increased pain sensitivity is not a characteristic effect of amphetamines, which might actually produce analgesic properties in certain contexts by increasing the release of endogenous opioids.

Hence, appetite suppression is the notable effect among the options given, aligning with the pharmacological profile of amphetamines.