A patient is prescribed 2 tablets, 3 times a day. Each tablet contains 200 mg of the drug. Calculate the total mass of the drug taken in one day in kilograms.

The prescription is "2 x 3", which means 2 tablets taken 3 times a day.

$$\text{Total tablets} = 2\, \text{tablets/dose} \times 3\, \text{doses/day}$$ $$\text{Total tablets} = 6\, \text{tablets/day}$$

Each tablet has a mass of $200\, \text{mg}$.

$$\text{Total mass in mg} = 6\, \text{tablets} \times 200\, \text{mg/tablet}$$ $$\text{Total mass in mg} = 1200\, \text{mg}$$

There are $1000\, \text{mg}$ in $1\, \text{g}$.

$$\text{Total mass in g} = \frac{1200\, \text{mg}}{1000\, \text{mg/g}}$$ $$\text{Total mass in g} = 1.2\, \text{g}$$

There are $1000\, \text{g}$ in $1\, \text{kg}$.

$$\text{Total mass in kg} = \frac{1.2\, \text{g}}{1000\, \text{g/kg}}$$ $$\text{Total mass in kg} = 0.0012\, \text{kg}$$

The total mass of the drug taken by the patient in one day is $\boxed{\text{0.0012 kg}}$.