Regeneration crisis

Regeneration crisis was a possible symptom of regeneration.

After having some of his regeneration energy siphoned into a newly regenerated future incarnation, who he had learnt had fallen asleep, the Third Doctor suggested that he was suffering from a regeneration crisis. (PROSE: The Christmas Inversion)

Following the Fourth Doctor's regeneration, the Tremas Master made a failed attempt to defeat the Fifth Doctor during his regeneration crisis. (PROSE: The Quantum Archangel, TV: Castrovalva)