Every once in a while, I get severe headaches complete with neck pain. I want to share a simple remedy that I now use every time.
Add a tbsp. of baking soda to 12 ounces of cold water and stir well. Drink the water as quickly as possible, then drink another 12 ounces of plain cold water.
I've noticed that this works every time, always without a OTC pain reliever. Alka-seltzer has baking soda in it, along with aspirin and caffeine.
Be careful with large amounts of baking soda such as this. It can affect your heart. It has sodium in it and should not be used with heart medications without your doctor's approval--just general advice for everyone. It can raise your blood pressure to be excessively high and/or give you heart arrhythmias (different types of abnormal heartbeats).
I'd also get the neck pain checked out. You may need a cardiac workup--simple tests but I'd do it. Severe headache combined with neck pain is NOT something to play around with. It could come from something as simple as poor posture at the computer or it could be a sign of heart trouble. Always pay attention to neck pain; it can be a sign of an impending heart attack and there is such a thing as a "silent" heart attack where you don't even know you're having one. Don't mean to scare you but just want to warn you.
Your headaches are relieved with 24 ounces of water and salt in the baking soda. Perhaps you are dehydrated and need electrolytes. Try drinking more water every day and see if that helps. It certainly won't hurt. But that wouldn't necessarily explain the neck pain. That is the biggest red flag I see.
When you ingest baking soda you are ingesting a lot of salt. Baking soda is sodium bicarbonate. Salt must be balanced with other minerals in your system or you can have all kinds of problems, most notably heart problems.
Baking soda neutralizes acids. Your headaches may be from excessive stomach acid--indigestion. As for the AlkaSeltzer, the baking soda neutralizes acids, aspirin is a pain reliever, and caffeine constricts blood vessels which lessens the amount of blood pulsating to your head, thus relieving the pressure that causes pain. Caffeine can also cause your heart to beat too fast, which you don't want, and it is a diuretic which will cause you to lose body fluids. If your headaches are from dehydration, you definitely don't want this. Most people are somewhat dehydrated all the time and don't know it. Thirst is a signal that the body needs more fluids fast. Try to drink enough water so that you never feel thirsty and your fluid level remains stable.
Please try to find the cause of the headaches with neck pain as soon as possible for your own good. Keep a log of what you ate before the pain began and what you were doing. It will help the doctor. (I studied nursing.)