Cough, frequent respiratory infections; impaired memory and learning; increased heart rate, anxiety; panic attacks; tolerance, addiction

Effects of Marijuana on the Brain
Researchers have found that THC changes the way in which sensory information gets into and is processed by the hippocampus.2 The hippocampus is a component of the brain's limbic system that is crucial for learning, memory, and the integration of sensory experiences with emotions and motivations. Neurons in the information processing system of the hippocampus and the activity of the nerve fibers in this region are suppressed by THC. In addition, researchers have discovered that learned behaviors, which depend on the hippocampus, also deteriorate via this mechanism.

Recent research findings also indicate that long-term use of marijuana produces changes in the brain similar to those seen after long-term use of other major drugs of abuse.

Effects on the Lungs
Someone who smokes marijuana regularly may have many of the same respiratory problems as tobacco smokers. These individuals may have daily cough and phlegm, symptoms of chronic bronchitis, and more frequent chest colds. Continuing to smoke marijuana can lead to abnormal functioning of lung tissue injured or destroyed by marijuana smoke.

Regardless of the THC content, the amount of tar inhaled by marijuana smokers and the level of carbon monoxide absorbed are three to five times greater than among tobacco smokers. This may be due to the marijuana users’ inhaling more deeply and holding the smoke in the lungs and because marijuana smoke is unfiltered.

Effects on Heart Rate and Blood Pressure
A marijuana user’s heart rate can increase when using marijuana alone. If the person uses cocaine at the same time, severe increases in heart rate and blood pressure can occur. The concern is that, in normal circumstances, an individual may smoke marijuana and inject cocaine and then do something physically stressful that may significantly increase the risk of overloading the cardiovascular system.

In one study, experienced marijuana and cocaine users were given marijuana alone, cocaine alone, and then a combination of both.6 The heart rate of the subjects in the study increased 29 beats per minute with marijuana alone and 32 beats per minute with cocaine alone. When the drugs were given together, the heart rate increased by 49 beats per minute, and the increased rate persisted for a longer time. The drugs were given with the subjects sitting quietly.

Effects of Heavy Marijuana Use on Learning and Social Behavior
A study of college students has shown that critical skills related to attention, memory, and learning are impaired among people who use marijuana heavily, even after discontinuing its use for at least 24 hours.7 Researchers compared 65 "heavy users," who had smoked marijuana a median of 29 of the past 30 days, and 64 "light users," who had smoked a median of 1 of the past 30 days. After a closely monitored 19- to 24-hour period of abstinence from marijuana and other illicit drugs and alcohol, the undergraduates were given several standard tests measuring aspects of attention, memory, and learning. Compared to the light users, heavy marijuana users made more errors and had more difficulty sustaining attention, shifting attention to meet the demands of changes in the environment, and in registering, processing, and using information. These findings suggest that the greater impairment among heavy users is likely due to an alteration of brain activity produced by marijuana.

Longitudinal research on marijuana use among young people below college age indicates those who used marijuana have lower achievement than the non-users, more acceptance of deviant behavior, more delinquent behavior and aggression, greater rebelliousness, poorer relationships with parents, and more associations with drug-using friends.

Research also shows more anger and more regressive behavior (thumb sucking, temper tantrums) in toddlers whose parents use marijuana than among the toddlers of non-using parents.

Effects on Pregnancy
Any drug of abuse can affect a mother's health during pregnancy, making it a time when expectant mothers should take special care of themselves. Drugs of abuse may interfere with proper nutrition and rest, which can affect good functioning of the immune system. Some studies have found that babies born to mothers who used marijuana during pregnancy were smaller than those born to mothers who did not use the drug. In general, smaller babies are more likely to develop health problems.

A nursing mother who uses marijuana passes some of the THC to the baby in her breast milk. Research indicates that the use of marijuana by a mother during the first month of breast-feeding can impair the infant's motor development (control of muscle movement).

Addictive Potential
A drug is addicting if it causes compulsive, often uncontrollable drug craving, seeking, and use, even in the face of negative health and social consequences. Animal studies suggest marijuana causes physical dependence, and some people report withdrawal symptoms.
Source: http://www.drugabuse.gov/Infofax/marijuana.html