Learn Dutch: Indien u een bijwerking ervaart, moet u de apotheker bellen. - In the event that you experience a side effect, you must call the pharmacist.

The word indien is a subordinating conjunction. Words like this introduce a dependent clause and always push the conjugated verb (in this case, ervaart) to the end of that clause.

Could I just use als instead of indien here?

Yes, grammatically als works perfectly and means the same thing. However, indien is much more formal. You will often see it in written instructions, official conditions, or medical texts like this one about side effects.

Why is it moet u instead of u moet in the second half of the sentence?

This is because of the V2 (verb second) rule. The entire first clause (Indien u een bijwerking ervaart) acts as the first building block of the whole sentence. To keep the main verb (moet) in the second position, it has to come right after the comma, pushing the subject (u) behind it.

Does the word bijwerking literally mean 'side work'?

Almost! Werking means 'effect' or 'operation' (from the verb werken). The prefix bij- is often used to mean 'extra' or 'secondary' (like in bijzaak, a secondary issue). Put them together, and a bijwerking is a secondary effect, or 'side effect'.