In humans, the AR gene, located on the X chromosome at Xq11-12, may affect penis size. The SRY gene located on the Y chromosome may have a role to play. Variance in size can often be attributed to de novo mutations. Deficiency of pituitary growth hormone or gonadotropins or mild degrees of androgen insensitivity can cause small penis size in males and can be addressed with growth hormone or testosterone treatment in early childhood. Conditions
An adult penis with an erect length of less than 7 cm or 2.76 inches but otherwise formed normally is referred to in a medical context as having the micropenis condition. The condition affects 0.6% of men. Some of the identifiable causes are deficiency of pituitary growth hormone or gonadotropins, mild degrees of androgen insensitivity, a variety of genetic syndromes and variations in certain homeobox genes. Some types of micropenis can be addressed with growth hormone or testosterone treatment in early childhood. Operations are also available to increase penis size in cases of micropenis in adults. Environmental influence
It has been suggested that differences in penis size between individuals are caused not only by genetics, but also by environmental factors such as culture, diet and chemical or pollution exposure. Endocrine disruption resulting from chemical exposure has been linked to genital deformation in both sexes (among many other problems). Chemicals from both synthetic (e.g., pesticides, anti-bacterial triclosan, plasticizers for plastics) and natural (e.g., chemicals found in tea tree oil and lavender oil) sources have been linked to various degrees of endocrine disruption.
Because of their effect on the part of the brain that regulates breathing, opioids can cause very slow or stopped breathing, during overdoses, leading to hypoxia  or death if left untreated. Hypoxia is typically caused by respiratory depression. The brain uses oxygen to regulate the homeostasis of the body. In animal studies, it was found that opioids act on specific regions of the central nervous system associated with respiratory regulation, including the medulla and pons. During cerebral hypoxia, there is a lack of sufficient oxygen supply to the brain. Prolonged lack of oxygenation from respiratory depression can lead to detrimental damage to the brain and spinal cord and can leave the person unable to walk or function normally, even if treatment with naloxone is given.Alcohol also causes respiratory depression and therefore when taken with opioids can increase the risk of respiratory depression and death.In young children, opioid overdose may not be apparent right away. This is due to absorption, distribution, and metabolism differences between young children and adults, and the higher amount of opioid ingestion per kilogram of body weight. Causes
Risk factors for opioid overdose include opioid dependence, injecting opioids, using high doses of opioids, and use together with alcohol, benzodiazepines, or cocaine. The risk is particularly high following detoxification. Dependence on prescription opioids can occur from their use to treat chronic pain. In young children an overdose is usually due to opioids that are intended for their parents, older siblings, or grandparents.