Effects of THC include relaxation, euforia, change in orientation, increased heart rate, lowered bloodpressure, impairment of coordination, concentration and short term memory.
Long term effects are less clear. If weed is smoked with tobacco, it can be physically addictive because of the nicotin. Also lung diseases can be the result of long term use. Alone, marihuana is not physically addictive.
Lethal dosage: there has never been a documented human fatality bc of weed. An estimated dosage for fatality in humans is about 1500 pounds of marihuana smoked in 15 minutes.
Effects of alcohol include relaxation, cheerfulness, blurred vision, coordination problems, dehydration. Higher dosages can have unconsiousness as effect, alcohol poisoning and death.
Long term effects include liver disease, cancer, damage to the central and peripheral nervous system, heart disease, long term memory loss, and a whole range of other diseases. Alcohol is physically addictive.
Lethal dosage: 40% of blood content is usually considered a lethal dosage, though it may be more or less dependent on the individual. Generally, a 100 pound man/woman consuming 9 or 10 standard drinks in less than an hour would be in the lethal dosage range.
So anyone saying you shouldn't do weed while buying you a drink should have his head examined