The phrase "an instant effect" is correct and usable in written English.
It can be used to describe a result or change that occurs immediately after an action or event.
Example: "The new medication provides an instant effect, alleviating pain within minutes."
Alternatives: "immediate impact" or "prompt result".
Brexit had an instant effect: the genie was out of the bottle.
For governments, the attractions were clear, because such departures had an instant effect on the unemployment figures.
"Porn has an instant effect on the human body and mind and the psyche, even if you disapprove of what you are seeing..
For three weeks, you're on Prozac, a drug that has an instant effect on your serotonin levels, and you feel as lousy as you did before.
Televised images of police turning fire hoses and dogs on demonstrators in Birmingham, Ala., had an instant effect on public opinion and national politics.
This, he said, played to the classic rural fears – crime, delinquency, the big city exporting immigration – and had an instant effect, "even in places like the wine-growing areas where people are relatively prosperous".
It is difficult to speculate on how relevant the reduction of FcγRs is for the physiological outcome of glucocorticoid treatment, since patients receiving an intraarticular injection of glucocorticoids experience an almost instant effect, but positive anti-inflammatory effects in the joint are seen several weeks after treatment.
Berlusconi wants to pass the bill as a decree, with instant effect, rather than "wasting time" with a normal parliamentary bill which would take months and be amended.
"That's what you can get with a mask: the instant effect".
If desire is a magic potion, with instant effect (see Tristan and Isolde), then love is a miracle whose effects become apparent only in time.
But also, eating a giant donut stuffed with cheesecake filling and topped with a citrus blueberry glaze has the instant effect of making you look like a kindergartner who has encountered sugar for the first time -- even if you're tackling it with a fork and knife.
