Marijuana grown and sold legally in Colorado is twice as potent as the illegal pot of past decades — and some modern weed is up to three times stronger, a new study shows, NBC News reported Monday. While vintage marijuana had around 10 percent THC (marijuana's active psychoactive chemical), modern-day Colorado dope averages near 18.7 percent, and it's up to 30 percent in some legal pot. "That was higher than expected," said Andy LaFrate, president of the lab that completed the study. But newer isn't necessarily better: The study also found that lots of legal weed has little medical value and often contains contamination like fungi. "We don't want to be alarmists and freak people out, but at the same time we have been finding some really dirty marijuana," LaFrate told NBC News.