Nothing will happen if he only took one. There’s a one in four chance that the pill was a dummy anyway, as most packets include seven tablets with no drugs in them (meant to maintain the habit of taking the pill while the ‘withdrawal bleed’ happens).
If he regularly took the ‘combined pill’, which contains oestrogen and progestogen hormones, it would have mild feminising effects, such as wider hips, softer skin and slight breast development.
The oestrogen dose is about a tenth of that taken by transgender women, but it’s a form of oestrogen associated with higher risks of deep vein thrombosis, so it wouldn’t be a good idea for anyone wanting to transition. Regularly taking a progestogen-only pill would have the main impact of reducing his sperm count and libido.
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