Possibly, but strychnine is a fairly rapid drug in its action. 
( Possibly ( , ( ( but strychnine ) ( ( is ( ( a ( ( fairly rapid ) drug ) ) ( in ( its action ) ) ) ) . ) ) ) )
(ROOT (S (ADVP (RB Possibly)) (, ,) (NP (IN but) (NN strychnine)) (VP (VBZ is) (NP (NP (DT a) (ADJP (RB fairly) (JJ rapid)) (NN drug)) (PP (IN in) (NP (PRP$ its) (NN action))))) (. .)))
Strychnine  is a drug that works very quickly. 
( Strychnine ( ( is ( ( a drug ) ( that ( works ( very quickly ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Strychnine)) (VP (VBZ is) (NP (NP (DT a) (NN drug)) (SBAR (WHNP (WDT that)) (S (VP (VBZ works) (ADVP (RB very) (RB quickly))))))) (. .)))