Is it safe to take Sudafed 30 minutes after taking DayQuil?

The quick answer is no. Sudafed is a decongestant and Dayquil contains a decongestant. They work by constricting blood vessels (thus shrinking/tightening the tissue in your sinuses/nose. But, they have the effect on your whole body. Too much can cause elevated blood pressure, rapid heartbeat, nervousness, shakiness, and can be very serious. It can be complicated if you are on any other medications that interact. If you did take the recommended dose of Dayquil and later wanted to take Sudafed (you ran out of Dayquil for example) I would wait at least 6 hours to take a recommended dose of Sudafed.