Yes, MARIHUANA is a Scrabble word!

Letter values in Scrabble US: M 3, A 1, R 1, I 1, H 4, U 1, A 1, N 1, A 1.

Is it a valid word?

MARIHUANA is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a strong-smelling plant from whose dried leaves a number of euphoriant and hallucinogenic drugs are prepared
the most commonly used illicit drug; considered a soft drug, it consists of the dried leaves of the hemp plant; smoked or chewed for euphoric effect

Also: cannabis sativa.