Our male cat was yowling several times a day. It was really annoying. According to vet it was due to heat cycle. We have been facing his loud yowling for months now, so vet asked us for neuter or anti heat injection. Today we took him to vet for anti-heat injection, but the cat is still yowling. May I know how much time it will take to start having an effect?
It is not clear from your description which drug was used for the "anti heat injection". But some of them can be partially effective for male cats. I would imagine it could take several weeks to become as effective as it can be, and it will need to be given repeatedly to have a long term impact.
Your best solution may be neuter as suggested by Flummox even the surgey may not be 100% effective see related Will a neutered cat continue to mount a female in heat?
Medroxyprogesterone acetate (MPA) is a synthetic progesterone. It is commonly used in humans as a form of birth control, and it may also be used in pets to suppress heat cycles. It can also be used to treat behavior problems such as aggression, spraying or marking.
Progesterone is a sex hormone found naturally in pets. At high doses, synthetic progesterone can prevent the ovaries from producing eggs. At smaller doses, it may slightly alter your pet’s behavior. It reduces sex drive in males, making them less likely to be aggressive towards other males. Also, if you have a male pet that roams in search of a mate, MPA can prevent this behavior. Source
Note, this is not a direct answer to your question, but will address the yowling.
If you have a male cat and do not intent to breed with it, please neuter him.
And I assume there is a female cat somewhere near that your cat would like to mate with. As long as she is in heat, he will yowl for / at her.