The short-acting (immediate-release) formulation of carbidopa/levodopa takes effect within about 20 to 50 minutes. The long-acting (extended-release) formulation starts to work closer to the 50-minute timeframe.

When switching away from a short-acting formulation, the first dose of a long-acting formulation may take 1 hour longer than the immediate-release to take effect.

The combination of carbidopa and levodopa can also be delivered directly to the stomach via a tube. This method typically involves an initial dose given over 10 to 30 minutes, followed by a continuous dose over 16 hours. The effect is strongest after 2.5 hours of the 16-hour infusion.

Eating lots of protein or acidic foods may delay the onset of these medications when consumed close together. You may need to take carbidopa/levodopa 1 to 2 hours before a meal and plan mealtimes to avoid overlap between doses and eating.

The duration of effect is longer for the extended-release formulation of carbidopa/levodopa than for the immediate-release formulation. The effects of extended-release carbidopa/levodopa should last between 4 to 6 hours. The effects of immediate-release carbidopa/levodopa last for approximately 2 to 3 hours less time — about 2 to 3 hours in total.