A multivitamin is not bad, but different vitamins need different other substances to be available for the body. Eg. some are soluble in water (like the B group) some, like vitamin D need fat to be solved in (and it’s good to acompany it with vitamin K to make D work as intended). Be careful with vitamin A - people in developed countries usually have too much and it can cause serious problems. It’s generally good to know what you’re taking and why. But if you just need a quick fix, a multivitamin is usually ok.