For most supplements, the best time to take them is whenever you will actually remember to. Consistency beats clever timing. But a handful do work better at a particular time or with a particular meal, and a few are worth keeping apart from each other. Here is a simple daily pattern that covers the ones where timing genuinely matters.
Morning, with or without food
Iron is best taken earlier in the day, on a fairly empty stomach if your gut tolerates it, and two hours away from coffee, tea, dairy and calcium. Vitamin C is optional: it does increase how much iron you absorb from a tablet, but trials found little or no difference in haemoglobin or ferritin. B vitamins can be mildly energising for some people, so morning suits them too.
With your main meal
The fat-soluble vitamins, vitamin D, E and K, plus omega-3, all absorb better with food that contains some fat. The largest meal of the day is the natural slot. Calcium, if you take a larger dose, absorbs better split across two smaller doses with meals than taken all at once.
Evening
Magnesium, particularly the gentler glycinate form, suits the evening, which is why it features in most sleep stacks; its evidence for sleep is modest, and the sleep guide says what the trial found. Anything you take specifically for sleep belongs here too.
The separations that matter
A few things override the general pattern. If you take thyroid medication, take it alone first thing and keep minerals at least four hours away, covered in the levothyroxine guide. If you are prescribed an antibiotic such as ciprofloxacin or doxycycline, take it at least two hours before, or six hours after, any iron, calcium, magnesium or zinc supplement, because a mineral taken first can cut how much of the antibiotic you absorb. And keep iron and calcium in different parts of the day, since calcium blunts iron. Beyond those, the clashes are minor and covered in what not to take together.
To check the timing of any supplement against a medication you take, the free checker will show you the result and the reasoning.