complicate
v.to make something more difficult to do, understand, or deal with. You use this when a situation becomes less simple because of new problems.
v.to make something more intricate, involved, or difficult to resolve. Often describes the introduction of new factors that hinder a process or diagnosis.
“Adding more rules will only complicate the game.”
“The patient's existing heart condition may complicate the upcoming surgery and require a longer recovery period.”
“The legal team warned that introducing a second witness at this late stage would likely complicate the trial and delay the final verdict by several months.”
The verb is transitive and takes a direct object; it is often used in the passive voice to describe a situation ('it is complicated').
Complicate is a transitive verb and does not take the preposition 'to' before its object.
Usage complicate matter
“They are incomplete, which complicates matters somewhat, but skilled anatomists can be reasonably confident about the complete bone sizes.”
“Complicating estimates, but improving the overall picture, is new technology such as onboard automatic start-stop systems in modern vehicles.”
Common in science writing
First attested in the early 17ᵗʰ century; borrowed from Latin complicātus, perfect passive participle of complicō (“to fold together”) (see -ate (verb-forming suffix) and -ate (adjective-forming suffix)), from con- (“together”) + plicō (“to fold, weave, knit”); see plaid, and compare complex. See also Middle English complicate (“involved”).
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.