It may have or has?
it is really confused me, because I found it many times had written like this:
- This trend could have negative consequences.
- Drug may have undesirable effects.

so why they used have instead of has. aren't 'trend' and 'drug' singular?

Question

It may have or has?
it is really confused me, because  I found it many times had written like this:
- This trend could have negative consequences.
- Drug may have undesirable effects.

so why they used have instead of has. aren't 'trend' and 'drug' singular?

Answer

It has nothing to do with the nouns being singular or plural, it's about the grammar rules when using more than one verb.

When you use a modal verb, such as "could" or "may," then the secondary verb must stay in it's original form. In this case you have the verb "have" and that verb remains as "have" in it's original form and will not change, due to the existence of a modal verb.

I hope my explanation is clear for you. If you have further questions, please ask me.

Answer

TREND and DRUG are singular, like you said.  But "have" refers to NEGATIVE CONSEQUENCES and UNDESIRABLE EFFECTS, which are both plural.