Generally, yes. I say generally because even though they are mostly natural products, if abused then can become dangerous. Supplements are often touted as safe, natural alternatives to more harmful substances such as anabolic steroids. This would be a correct assumption, but some caution still needs to be taken. Some of today's most effective supplements, such as pro-hormones and thermogenic products are very effective at producing results, but many people feel that because they are 'natural' products, then they are 100% safe. This is where people can get into trouble. Supplement manufacturers make recommendations on dosages from sounds judgements based on scientific research and real world applications.
To ensure that your supplement program is not only effective but safe as well, stick to the directions on the bottle. If you have more questions about a particular product, you can always contact the manufacturer.