If you take the contraceptive pill, there is one supplement that genuinely matters and a few smaller patterns worth knowing. The headline is simple: one popular herbal supplement can make the pill less reliable. Most other supplements are fine alongside it.
The one that can cause a pregnancy: St John's Wort
St John's Wort, taken for low mood, speeds up the liver enzymes that clear the hormones in the combined pill. The result is lower hormone levels, and several randomised trials and real-world reports document reduced contraceptive reliability and unplanned pregnancies as a consequence. This is a genuine hard stop on both the combined pill and the progestogen-only pill. If you are on the pill for contraception and considering St John's Wort for your mood, that is a conversation to have with your GP first, because there are options that do not undermine your contraception.
The smaller pattern: the pill nudges a few nutrients down
Long-term use of the combined pill is associated, at a group level, with slightly lower levels of several nutrients: folate, B6, B12 and the other B vitamins, vitamin C, zinc and selenium. The effect is modest and not something most people will notice, but it is the reason a standard multivitamin or B-complex is a reasonable, low-stakes addition if you have been on the pill for years. Nothing dramatic, just topping up the small gaps.
What is fine
The everyday supplements people ask about, including magnesium, vitamin D and omega-3, sit comfortably alongside the pill with no interaction worth worrying about.
The simple version
Leave St John's Wort off the list while you rely on the pill for contraception. Treat a multivitamin as a sensible top-up rather than a necessity. To check a specific supplement against your contraceptive, the free checker will show you the call and the reasoning. The same St John's Wort warning applies on HRT, for the same reason.