OP:
First,
Reducing Viagra Headaches. Read that.
Second, Viagra is only one formulation (Sildenafil) in the PDE5 family. If it gives you headaches, try Tadalafil, or a different one.
Third, you can get liquid Tadalafil without a prescription from places like Rasa Research online - or other formulations.
Remember that PDE5 drugs enlarge your arteries - when you then elevate your heart-rate, you're pushing blood through faster...which is why you have a headache. Take a blood thinner; aspirin, etc.