A decrease in semen volume and thickness can be caused by lifestyle factors like dehydration, stress, or infrequent ejaculation, as well as medical issues like hormonal imbalances or prostate changes. There is no single medication to take without a proper diagnosis. Initially, I recommend staying well-hydrated, maintaining a balanced diet, and taking supplements like Zinc, Vitamin D, and Coenzyme Q10, which support reproductive health. Most importantly, please consult a urologist. They can perform a semen analysis and blood tests to pinpoint the exact cause and prescribe targeted treatments if necessary.