I'm neither doctor nor scientist (of any field) but I can assure you, with nearly 20 years of smoking weed, I have never foamed at the mouth nor have I heard such a thing to be common amongst weed smokers (did you mean drooling?). If you are certain its the 'weed' doing it, then stop smoking that weed and find another provider. Weed can be sprayed with a host of chemicals (to add weight to the counts [price will go up] and to add intensity to the bud) and grown with a number of chemical fertilizers that can negatively affect the human body (I do not know if foaming could be a by-product). Mold and other bacteria strains can flourish on inappropriately handled/grown/transported/stored 'weed'/'green' as well (moldy green can cause lung infections). Are you on any medications or taking any pills recreational? (these, mixed with 'green' can really harm your body/mind and cause negative reactions). If the foaming persists (with or without smoking green) see a doctor; foaming at the mouth is a health warning! Many people suffer diseases in America (10 million cited on one site) that are linked to foaming at the mouth. You might want to do a quick search typing (health foaming at the mouth).