Cigarette smoking is associated with modest reduction in sperm concentration(13-17%), motility and morphology. Smoking may alter hormone levels in males. Limited studies suggested smoking to be mutagenic to human spermatozoa and lead to cancer, birth defect and genetic diseases in offspring.

Excessive alcohol intake is associated with direct testicular toxicity. Alcohol abuse and alcoholism are associated with hypothalmic-pituitary-ovarian dysfunction resulting in amenorrhea (absence of menses), anovulation (lack of ovulation), luteal phase dysfunction (abnormal development of the endometrial lining) and hyperprolactinemia. The adverse reproductive consequences of alcohol range from infertility and increased risk for spontaneous abortion to impaired fetal growth and development.

In women, the ovarian reserve and quality of eggs deteriorate with smoking. Tobacco contains hundreds of substances, many of them toxic. One study showed that the chance for an IVF pregnancy was 2.7 times higher for women who have never smoked as compared to women that do (or have previously). The same study showed that if the woman smoked for over 5 years, the risk was increased to 4.8

There is also some evidence that male smoking decreases success rates with IVF. It is not known whether this effect would be caused directly by the sperm, or whether it represents a negative effect on the egg and embryo quality through passive exposure (second hand exposure).