Thanks for your query.Based on the facts that you have posted it appears that you have penile discharge
since few days. You have not mentioned the reason for taking Allopurinol? Was it advised by a Dr or you have started taking yourself.This drug has to be taken under supervision of a qualified Dr.
In absence of history of sex or any other urinary symptoms it can not be STD
.neither it is due to side effect of the drug. ,but it is due to an excessive secretion of inflamed periurethral glans penis
located in urethra.
Take broad spectrum antibiotics for couple of days .It is bound to get resolved within 2-3 weeks.