Why do some drugs require you to take them in an "upright position"?
Metamucil is taken as a drink. You're more likely to aspirate versus swallow when drinking supine if you're ill with impaired swallowing reflexes.

It's also recommended that you take bisphophonates standing up so that gravity, and a glass of water, make sure the drug passes into the stomach. It can be highly corrosive if it gets stuck in the esophagus.