Vincristine

Vitamin C enhanced the drug's effect in vivo (Lamson & Brignall 2000) — potentially beneficial but difficult to assess.

PS-341 (BORTEZOMIB, VELCADE)

This is a proteasome inhibitor approved by the US FDA for the treatment of patients with relapsed multiple myeloma. Vitamin C inactivated drug activity in vitro (Zou et al 2006) —avoid.

Clinical note — Does vitamin C interact with the OCP?

In 1981 a case was reported of a woman who had experienced heavy breakthrough bleeding as a result of stopping vitamin C supplementation while taking the OCP (Morris et al 1981). At the time, it was suspected that vitamin C in high doses increases the bioavailability of oestrogen and raises blood concentrations due to competition for sulfation (resulting in reduced drug metabolism) (Back & Orme 1990). Therefore, ceasing supplement use would have the opposite effect and potentially cause breakthrough bleeding, as reported in this case. Since then, further investigation has been conducted to investigate whether this interaction is clinically significant. In 1993 a placebo-controlled study was conducted with 37 women and found that 1000 mg of vitamin C does not lead to an increased systemic bioavailability of ethinyl oestradiol, and therefore the purported interaction is unlikely to be of any clinical importance (Zamah et al 1993).

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