Cancer — Marijuana smokers probably are at increased risk for lung cancer, although the magnitude of risk has not been well quantified. The absolute risk of lung cancer that a given individual accrues likely depends upon the magnitude and duration of drug use, the amount of adulterants coingested, and whether exposure to concomitant carcinogens (such as tobacco smoke) is present. Several reports have documented histologic and molecular changes in the bronchial epithelium of marijuana smokers that are similar to the metaplastic premalignant alterations seen among tobacco smokers. A systematic review of 19 studies did not detect a significant association between lung cancer and marijuana use, after adjusting for tobacco smoking. The negative association may reflect insufficient lag time for cancer development in the observational studies, given the relatively young age of study participants.

A review of clinical studies published in 1997 was unable to document a relationship between marijuana use and the development of head and neck squamous cell carcinoma (HNSCC). However, a later study suggested that marijuana use may modestly increase the risk of HNSCC, an effect that is magnified by cigarette smoking. A population-based case-control study did not find an association between marijuana use and oral squamous cell carcinoma.

Habitual use of marijuana may be a risk factor for transitional cell cancer of the bladder, based on data from a case control study in a group of veterans younger than 60 years of age. These are preliminary data, potentially confounded by a high rate of nicotine use in this population.

Reproductive effects — In men, marijuana causes decreased serum testosterone levels, sperm count, and sperm motility. This may lead to decreased libido, impotence, and gynecomastia. An increased risk of infertility may result from changes in semen characteristics seen with marijuana smoking.

In women, chronic marijuana use causes shorter menstrual cycles and increased prolactin levels. The latter may cause galactorrhea.

THC accumulates in breast milk and crosses the placenta . This can lead to low birthweight babies and abnormal reflexes and responses in newborns.

Chronic obstructive pulmonary disease — The association between tobacco smoking and chronic obstructive pulmonary disease (COPD) has been established. Short-term marijuana administration is associated with bronchodilation. Longterm smoking is associated with many symptoms of obstructive pulmonary disease: airflow obstruction, chronic cough, bronchitis, and decreased exercise tolerance . A link between marijuana use and an unusual form of COPD characterized by large lung bullae is also suspected, and may be associated with spontaneous pneumothorax .. However, a consistent dose-response relationship between long-term marijuana smoking and FEV1/FVC ratio, diffusion capacity, or airway hyperreactivity, was not found in a systematic review of 34 studies that took into account study quality, and adjustments for tobacco smoking when data was available.

Regular smokers of three to four marijuana cigarettes per day experience cough, wheeze, and sputum production and exhibit histologic abnormalities equivalent to those who smoke approximately 20 tobacco cigarettes per day. This disparity may be due in part to the different manner in which marijuana and tobacco cigarettes are smoked. On average, inhalation from a marijuana cigarette delivers almost twice as much smoke, the depth of inspiration is one-third longer, and the breath holding time is four times longer than when a tobacco cigarette is smoked.