Reduced testosterone levels could be linked to age. However, while they tend to decline with age, they stay within normal limits in most men. And there's often no correlation between testosterone levels and erectile dysfunction. If your tests show very low testosterone levels, the hormone can be taken by gel application or injection - only under the recommendation of a doctor. With gels, you will have to apply it on your upper arms, shoulders or thighs while the injection is given in the buttocks. After application of the gel, you need to avoid skin contact with others for a few hours, especially women, as the gel could cause acne or excess hair growth. The injection has its effects for a few days, during which you will see mood swings and change in energy levels.