What if I am taking other drugs? Erectile dysfunction medications require a prescription from a licensed provider. Take tadalafil exactly as directed. Tadalafil belongs to a group of medicines called phosphodiesterase 5 PDE5 inhibitors. Tell your healthcare provider that you take Cialis.For the as-needed dose, plan to take Cialis at least 30 minutes before sexual activity, but bear in mind it may take as long as 2 hours to take effect. Also, Cialis requires sexual stimulation to. Side Effects. Headache, stomach upset, back pain, muscle pain, stuffy nose, flushing, or dizziness may occur. If any of these effects last or get worse, tell your doctor or pharmacist promptly. To. Buying prescription drugs online is easy with Canada Pharmacy. All you need to do is search for the brand or generic prescription drug using the boxes above and select your medication and checkout. Save up to 80% compared local US pharmacies and also take advantage of our price match guarantee. Purchasing Canadian prescription drugs .