Hydroquinone, aka benzene-1,4-diol or quinol, is an aromatic organic compound that is a type of phenol, a derivative of benzene, having the chemical formula C6H4(OH)2. Its chemical structure features two hydroxyl groups bonded to a benzene ring in a para position. It is a white granular solid. Substituted derivatives of this parent compound are also referred to as hydroquinones. The name "hydroquinone" was coined by Friedrich Wöhler in 1843. In human medicine, hydroquinone is used as a topical application in skin whitening to reduce the color of skin. It does not have the same predisposition to cause dermatitis as metol does. In 2006, the United States Food and Drug Administration revoked its previous approval of hydroquinone and proposed a ban on all over-the-counter preparations. The FDA stated that hydroquinone cannot be ruled out as a potential carcinogen. This conclusion was reached based on the extent of absorption in humans and the incidence of neoplasms in rats in several studies where adult rats were found to have increased rates of tumours, including thyroid follicular cell hyperplasias, anisokaryosis (variation in nuclei sizes), mononuclear cell leukemia, hepatocellular adenomas and renal tubule cell adenomas. One of the components in TRI-LUMA Cream, hydroquinone, is a depigmenting agent, and may interrupt one or more steps in the tyrosine-tyrosinase pathway of melanin synthesis. However, the mechanism of action of the active ingredients in TRI-LUMA Cream in the treatment of melasma is unknown.

Apaziquone (EOquin, EO9) is an indolequinone that is a bioreductive prodrug and a chemical analog of the older chemotherapeutic agent mitomycin C. In hypoxic cells, such as those on the inner surface of the urinary bladder, apaziquone is converted to active metabolites by intracellular reductases (such as NQO1). The active metabolites alkylate DNA and lead to apoptosis. In animal tumour models, EO9 was inactive against the P388 murine leukaemia but exhibited anti-tumour activity against human tumour xenografts and the generally chemo-resistant murine adenocarcinomas of the colon (MAC) tumours. Initial evidence that in vivo response correlated with NQO1 activity. Apaziquone was selected for clinical evaluation based upon its novel mechanism of action (which was distinct from MMC), its preferential activity against cells derived from solid tumours in vitro and in vivo, its ability to target both aerobic and hypoxic cells and the lack of myelosuppression in mice and rats. Apaziquone has been applied in clinical studies sponsored by Spectrum Pharmaceuticals and Allergan, Inc. for the treatment of superficial (non-muscle invasive) bladder cancer. However, the US-FDA determined that it was not statistically effective.

Hydroquinone monoacetate (also known as 4-acetoxyphenol), an FDA-approved natural compound, was studied as a potent activator of the NRF2-ARE pathway in motor neurons and as a α-synuclein modulator that can possibly prevent or slow down the onset or progression of Parkinson's disease. In addition, was evaluated the role of 4-acetoxyphenol in age-related macular degeneration (AMD), a degenerative disorder of the central retina, which is the leading cause of irreversible central vision loss in elderly populations in developed countries. It was shown, that 4-acetoxyphenol protected human retina pigment epithelium cell lines from oxidative stress-induced necrosis through upregulation of NQO1 and HO-1 genes by stabilization of NRF2, a transcription factor and a master regulator of many antioxidant/detoxification genes. Thus, 4-Acetoxyphenol could be further evaluated for its potential use in preventing AMD progression.