sorry, but I do not understand why to use this expression in:
This product is to be taken at night.
Why not "this product must be taken at night"?
You are talking about the form “BE TO + INFINITIVE”.
There is a series of rules:
1. When we talk about official or formal arrangements.
2. To offer formal instructions (medicine, chemicals, etc.).
3. To give orders.
4. In news reports to refer to future events.
5. In particular, you must consider that this form is used when talking about events or things that can be controlled or altered by humans. So natural events beyond our control cannot be described using this form.
Therefore, we cannot say:
The hurricane is to strike land tomorrow.
The hurricane is going to strike land tomorrow.
6. In if-clauses to express the idea that something must take place first before something else in the if-clause.
If social justice is to be done, diverse economic issues must be addressed and solved.
José Manuel Rosón Bravo