If someone takes adderall (dextroamphetamine and racemic amphetamine) then smokes weed after would the side effects of both drugs be fatal since one is a downer and one is an upper ?

Not fatal: Adderrall is a stimulate and marijuana is a relaxant. The effects of mixing are not that dramatic and will vary in individuals, but both only have short term effects. The effects of taking Adderall (dextroamphetamine and racemic amphetamine) while smoking\tmarijuana? It can effect the heart. Mixing drugs can always heart effects and may cause depression.

RISKY: Combining medications/drugs that effect the brain is risky. Talk honestly with your doctor about your use of medication and drugs.

Not fatal, but dumb: The combination is not likely to be fatal, but there is no logic to it. The marijuana clouds consciousness, when you were supposedly trying to increase focus by taking adderall (dextroamphetamine and racemic amphetamine). Please don't turn yourself into a chemical experiment this way. Your brain is a precious thing to waste. Stay healthy. And remember that both Adderall (dextroamphetamine and racemic amphetamine) and marijuana increase heart rate. Why chance it?

Could be: Yes, they can be. Both work on the brain and either can be fatal in certain individuals. Also, this combo can cause severe interactions that may not be fatal but may cause psychosis, hallucinations, uncontrolled violence, etc.