頭痛を軽くみてはいけません。

headache

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

light (i.e. not heavy); feeling light (i.e. offering little resistance, moving easily); light (i.e. of foot); nimble; agile; non-serious; minor; unimportant; trivial; slight; small; gentle; soft; easy; simple; indiscriminate

to see; to look; to watch; to view; to observe; to look over; to look on; to assess; to examine; to judge; to look after; to keep an eye on; to take care of; to view (e.g. flowers, movie); to try; to try out; to see that...; to find that...

if ... then; when; and; why don't you...?; the more (one does something); because; since