An active placebo is a placebo

Placebo

A placebo is a simulated or otherwise medically ineffectual treatment for a disease or other medical condition intended to deceive the recipient...

 that mimics the common side effects of the drug under study.

Nomenclature

According to a 1965 paper, the term "fake placebo" (German: Kaschiertes Placebo) was suggested in a 1959 paper published in German.

Example

Morphine

Morphine

Morphine is a potent opiate analgesic medication and is considered to be the prototypical opioid. It was first isolated in 1804 by Friedrich Sertürner, first distributed by same in 1817, and first commercially sold by Merck in 1827, which at the time was a single small chemists' shop. It was more...

 and gabapentin

Gabapentin

Gabapentin is a pharmaceutical drug, specifically a GABA analogue. It was originally developed for the treatment of epilepsy, and currently is also used to relieve neuropathic pain...

 are painkillers with the common side effects of sleepiness and dizziness. In a 2005 study assessing the effects of these painkillers on neuropathic pain

Neuropathic pain

Neuropathic pain results from lesions or diseases affecting the somatosensory system. It may be associated with abnormal sensations called dysesthesia, which occur spontaneously and allodynia that occurs in response to external stimuli. Neuropathic pain may have continuous and/or episodic ...

, Lorazepam

Lorazepam

Lorazepam is a high-potency short-to-intermediate-acting 3-hydroxy benzodiazepine drug that has all five intrinsic benzodiazepine effects: anxiolytic, amnesic, sedative/hypnotic, anticonvulsant, antiemetic and muscle relaxant...

was chosen as an active placebo because it is not a painkiller but it does cause sleepiness and can cause dizziness.