Reference for Niddah 33:20

וכן אמר רבי יוחנן

'And blood-letting followed immediately by intercourse'. [This should be avoided] because a Master said: If a man has intercourse immediately after being bled, he will have feeble<span class="x" onmousemove="('comment',' Or 'nervous'. ');">¹⁶</span>

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