How many letters does the top answer for “increasing (or tending to increase) blood pressure” have?

The most common answer is pressor (7 letters).

Crossword clue: increasing (or tending to increase) blood pressure

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

pressor 7 letters wn-def

any agent that causes a narrowing of an opening of a blood vessel: cold or stress or nicotine or epinephrine or norepine